Add yeast documentation

Covers architecture, query language, template language (tree!/trees!/rule!),
capture semantics, fresh identifiers, and extractor integration.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

shared/yeast/doc/yeast.md

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+# YEAST — YEAST Elaborates Abstract Syntax Trees
+
+YEAST is a framework for transforming tree-sitter parse trees before they are
+extracted into a CodeQL database. It sits between the tree-sitter parser and
+the TRAP extractor, rewriting parts of the AST according to declarative rules.
+
+## Motivation
+
+Tree-sitter grammars describe the **concrete syntax** of a language — every
+keyword, operator, and punctuation token appears in the parse tree. CodeQL
+analyses often prefer a **simplified abstract syntax** where syntactic sugar
+has been removed. YEAST bridges this gap by desugaring the tree-sitter output
+into a cleaner form before extraction.
+
+For example, Ruby's `for x in list do ... end` is syntactic sugar for
+`list.each { |x| ... }`. A YEAST rule can rewrite the former into the latter
+so that CodeQL queries only need to reason about the `.each` form.
+
+## Architecture
+
+```
+Source code
+    │
+    ▼
+┌──────────────┐
+│  tree-sitter │  Parse source into a concrete syntax tree
+│    parser    │
+└──────┬───────┘
+       │ tree_sitter::Tree
+       ▼
+┌──────────────┐
+│    YEAST     │  Apply desugaring rules, producing a new AST
+│   Runner     │
+└──────┬───────┘
+       │ yeast::Ast
+       ▼
+┌──────────────┐
+│    TRAP      │  Walk the (possibly rewritten) AST and emit TRAP tuples
+│  extractor   │
+└──────────────┘
+```
+
+The entry point is `extract_and_desugar()` in the shared tree-sitter
+extractor, which passes a set of rules to the YEAST `Runner`. The original
+`extract()` function passes empty rules, leaving the tree unchanged.
+
+## How desugaring works
+
+A YEAST `Rule` has two parts:
+
+1. A **query** that matches nodes in the AST using a tree-sitter-inspired
+   pattern language.
+2. A **transform** that produces replacement nodes from the match captures.
+
+The `Runner` applies rules by walking the tree top-down. At each node, it
+tries each rule in order. If a rule's query matches, the node is replaced by
+the transform's output, and the rules are re-applied to the result. If no
+rule matches, the node is kept and its children are processed recursively.
+
+A rule can replace one node with zero nodes (deletion), one node (rewriting),
+or multiple nodes (expansion).
+
+## Query language
+
+Queries use a syntax inspired by
+[tree-sitter queries](https://tree-sitter.github.io/tree-sitter/using-parsers/queries/index.html),
+written inside the `yeast::query!()` proc macro.
+
+### Node patterns
+
+```rust
+// Match any named node
+(_)
+
+// Match a node of a specific kind
+(assignment)
+
+// Match an unnamed token by its text
+("end")
+```
+
+### Fields
+
+```rust
+// Match a node with specific fields
+(assignment
+    left: (identifier) @lhs
+    right: (_) @rhs
+)
+```
+
+Fields are matched by name. Unmentioned fields are ignored — the pattern
+`(assignment left: (_) @x)` matches any `assignment` node regardless of
+what's in `right`.
+
+### Captures
+
+Captures bind matched nodes to names for use in the transform. A capture
+`@name` always follows the pattern it captures:
+
+```rust
+(identifier) @name          // capture an identifier node
+(_) @value                  // capture any named node
+(identifier)* @items        // capture each repeated match
+```
+
+### Unnamed children
+
+Patterns that appear after all named fields match unnamed (positional)
+children. Named node patterns like `(_)` automatically skip unnamed tokens
+(keywords, operators, punctuation), matching tree-sitter semantics:
+
+```rust
+(for
+    pattern: (_) @pat             // named field
+    value: (in (_) @val)          // "in" token is skipped automatically
+    body: (do (_)* @body)         // "do" and "end" tokens skipped
+)
+```
+
+### Repetitions
+
+```rust
+(_)*                   // zero or more
+(_)+                   // one or more
+(_)?                   // zero or one
+(identifier)* @names   // capture each repeated match
+```
+
+## Template language
+
+Templates construct new AST nodes using the `tree!` and `trees!` macros.
+All children in a template must be in named fields — output AST nodes are
+always fully fielded.
+
+When used inside a `rule!` macro, the context is implicit — no explicit
+`BuildCtx` argument is needed. When used standalone, they take a `BuildCtx`
+as the first argument:
+
+```rust
+// Inside rule! — implicit context, captures are Rust variables
+yeast::rule!(
+    (assignment left: (_) @left right: (_) @right)
+    =>
+    (assignment left: {right} right: {left})
+);
+
+// Standalone — explicit context
+let fresh = yeast::tree_builder::FreshScope::new();
+let mut ctx = BuildCtx::new(ast, &captures, &fresh);
+let id = yeast::tree!(ctx,
+    (assignment
+        left: {ctx.capture("lhs")}
+        right: {ctx.capture("rhs")}
+    )
+);
+```
+
+### `tree!` — build a single node
+
+`tree!(...)` returns a single node `Id`:
+
+```rust
+yeast::tree!(ctx,
+    (assignment
+        left: {ctx.capture("lhs")}
+        right: {ctx.capture("rhs")}
+    )
+)
+```
+
+### `trees!` — build multiple nodes
+
+`trees!(...)` returns `Vec<Id>`:
+
+```rust
+yeast::trees!(ctx,
+    (assignment left: {tmp} right: {right})
+    {..body}
+)
+```
+
+### Literal nodes
+
+`(kind "text")` creates a leaf node with fixed text content:
+
+```rust
+(identifier "each")          // an identifier node whose text is "each"
+```
+
+### Computed literals
+
+`(kind #{expr})` creates a leaf node whose content is `expr.to_string()`:
+
+```rust
+(integer #{i})               // an integer node with the value of i
+(identifier #{name})         // an identifier from a Rust variable
+```
+
+### Fresh identifiers
+
+`(kind $name)` creates a leaf node with an auto-generated unique name. All
+occurrences of the same `$name` within one `BuildCtx` share the same value:
+
+```rust
+(block
+    parameters: (block_parameters
+        (identifier $tmp)         // generates e.g. "$tmp-0"
+    )
+    body: (block_body
+        (assignment
+            left: {pat}
+            right: (identifier $tmp)   // same "$tmp-0" value
+        )
+    )
+)
+```
+
+### Embedded Rust expressions
+
+`{expr}` embeds a Rust expression that returns a single node `Id`:
+
+```rust
+(assignment
+    left: {some_node_id}       // insert a pre-built node
+    right: {rhs}               // insert a captured value (inside rule!)
+)
+```
+
+`{..expr}` splices a `Vec<Id>` (or any iterable of `Id`):
+
+```rust
+yeast::trees!(ctx,
+    (assignment left: {tmp} right: {right})
+    {..extra_nodes}                        // splice a Vec<Id>
+)
+```
+
+Inside `rule!`, captures are Rust variables, so `{name}` inserts a
+single capture (`Id`) and `{..name}` splices a repeated capture
+(`Vec<Id>`).
+
+## Complete example: for-loop desugaring
+
+This rule rewrites Ruby's `for pat in val do body end` into
+`val.each { |tmp| pat = tmp; body }`:
+
+```rust
+let for_rule = yeast::rule!(
+    (for
+        pattern: (_) @pat
+        value: (in (_) @val)
+        body: (do (_)* @body)
+    )
+    =>
+    (call
+        receiver: {val}
+        method: (identifier "each")
+        block: (block
+            parameters: (block_parameters
+                (identifier $tmp)
+            )
+            body: (block_body
+                (assignment
+                    left: {pat}
+                    right: (identifier $tmp)
+                )
+                {..body}
+            )
+        )
+    )
+);
+```
+
+Captures from the query (`@pat`, `@val`, `@body`) become Rust variables
+automatically: single captures bind as `Id`, repeated captures (after
+`*` or `+`) as `Vec<Id>`, and optional captures (after `?`) as
+`Option<Id>`.
+
+## The `rule!` macro
+
+`rule!` combines a query and a transform into a single declaration:
+
+```rust
+// Full template form
+yeast::rule!(
+    (query_pattern field: (_) @capture)
+    =>
+    (output_template field: {capture})
+)
+
+// Shorthand form — captures become fields on the output node
+yeast::rule!(
+    (query_pattern field: (_) @capture)
+    => output_kind
+)
+```
+
+The shorthand `=> kind` form auto-generates the template, mapping each
+capture name to a field of the same name on the output node.
+
+## Integration with the extractor
+
+YEAST integrates with the shared tree-sitter extractor via two mechanisms:
+
+1. **`extract_and_desugar()`** — like `extract()`, but takes a
+   `Vec<yeast::Rule>` to apply before TRAP extraction.
+
+2. **`LanguageSpec::output_node_types`** — when desugaring produces an AST
+   with different node types than the tree-sitter grammar, this field points
+   to a separate `node-types.json` describing the output schema.
+
+Languages that don't use desugaring simply call `extract()`, which passes
+empty rules internally.