M3: Closures (= by-value / := by-reference capture)

[assapir]

What

Adds lexical closures to Quilon. The capture mode is inferred entirely from the binding operator — no capture list, no marker — mirroring the language's existing mutability rule:

• a name bound with = is captured by value (a frozen, read-only snapshot);
• a name bound with := is captured by reference — a single shared GC-boxed cell, so writes escape the closure and persist/accumulate across calls.

Closures are monomorphic in M3 (concrete-typed params and captures). Capturing a polymorphic value, generic closures, passing a closure as a function parameter, and returning a closure from a function are deferred to M4; a closure used in an unsupported position is rejected at compile time, never miscompiled.

How (pipeline)

• AST (nodes.rs): new Expr::Lambda function-literal node.
• Parser (ast_parser.rs): lambda expressions (x => …, (a, b) => …, () => …, with optional -> Type). A one-shot no_lambda guard prevents a for-loop collection (for n <- xs => body) from being mis-parsed as xs => body. Parameter parsing is shared via parse_param_list.
• Checker (checker.rs): check_lambda infers Type::Function; the body is checked in a scope layered over the enclosing environment, so captures are visible and the immutability gate still applies inside closures.
• Codegen (generator.rs): a closure value is { ptr fn, ptr env }; the lifted top-level function takes the captured environment as a trailing pointer. = captures are copied by value into the env; := captures are heap-boxed via the existing __alloc intrinsic and shared (writes escape, escape-safe). A non-capturing nested function is emitted as a plain module function so it can recurse; captures thread through arbitrary nesting depth, and a closure value may itself be captured by another closure and called.
• Capture analysis (ast/captures.rs): free-variable computation parameterized by the enclosing scope, so a := to an outer name is recognized as a reassignment of a captured cell (a use) rather than a fresh local.

Tests / docs

• examples/closures.ql — a counter that mutates a :=-captured cell across calls (writes escape) plus an adder that captures an = binding by value; ^ returns exit code 42. Wired into tests/examples_test.rs, which runs it under the JIT and native AOT (clang + gcc) and asserts all three agree.
• tests/closures_test.rs — 12 run tests: := accumulation, frozen = snapshot, shared cells, mixed capture, parameter shadowing, immutability preserved through capture, non-capturing nested recursion, capturing-and-calling another closure, and multi-level capture.
• LANGUAGE.md — new Closures section, feature-matrix entry, and the M4 boundary.

Green gate

• cargo build ✅
• cargo test ✅ (incl. the native-AOT examples gate, run with clang + gcc — not JIT only)
• cargo fmt --check ✅
• cargo clippy --all-targets -- -D warnings ✅

/code-review and /simplify were run; findings addressed (notably: nested-recursion regression fixed, multi-level mutable capture sharing fixed, capturing-a-closure call fixed, and duplicated signature/return-type/call-result logic factored into shared helpers).

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