Iter 14f: Boehm conservative GC

Decision 9 ships. Through Iter 14e every ADT box, lambda env, and
closure pair was leaked. This iter substitutes GC_malloc for malloc
in all four IR allocation sites and links -lgc. No language change,
no AST change, no schema change.

Diff: 5 files modified, ~30 LOC net.
- codegen/lib.rs: 4 substitutions @malloc -> @GC_malloc.
- ail/main.rs: .arg("-lgc") in the clang invocation.
- 5 IR snapshot files: mechanical s/@malloc/@GC_malloc/, 9
  occurrences. IR is bit-identical to pre-14f modulo this
  substitution — exactly Decision 9's promise.
- e2e.rs: new test gc_handles_recursive_list_construction.
- examples/gc_stress.{ailx,ail.json}: new fixture, builds a 50-
  element list via recursive Cons, sums it (1275).

Hash invariance verified: every existing fixture def hash
unchanged (codegen and link line are downstream of canonical
bytes; AST didn't move).

Tests 79 -> 80, all green. Existing 79 byte-identical stdout.
gc_stress -> 1275. list_map_poly -> 2/3/4 unchanged. sort
sorted-list unchanged. cargo doc 0 warnings.

GC notes (pertinent to future work):
- GC_INIT() not needed on Arch libgc 1.5.6 (auto-init via
  __attribute__((constructor))).
- No conservative-scan over-retention observed.
- -lgc alone sufficient for link (pthread/dl transitive).

Pattern-shape note from gc_stress fixture writing: the post-14d
"if-then-else" replacement is `(match (app == n 0) (case
(pat-lit true) ...) (case (pat-wild) ...))`. Three lines for
what `if` used to do in one, but uniform with the language.
Worth flagging for the stdlib brief.

Language is feature-complete enough for stdlib. The three
blockers identified at the 14b boundary (redundancy 14d, tail
calls 14e, GC 14f) are all done. Plan 15a: first stdlib module
std_list.ailx with length/append/reverse/map/filter/fold_left/
fold_right/head/tail/is_empty.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/JOURNAL.md

@@ -2002,6 +2002,102 @@ Anything else (records as a primitive, nested patterns, local
 recursive let, type classes) can layer on later without forcing
 stdlib rewrites.
 
+## Iter 14f — Boehm conservative GC
+
+Decision 9 ships. Through Iter 14e every ADT box, lambda env,
+and closure pair was leaked. This iter substitutes
+`GC_malloc` for `malloc` in all four IR allocation sites and
+links `-lgc`. No language change, no AST change, no schema
+change.
+
+**Diff: 5 files, ~30 LOC net.**
+
+- `crates/ailang-codegen/src/lib.rs`: 4× `@malloc` → `@GC_malloc`
+  (declare line + 3 call sites: `lower_ctor`, `lower_lambda`'s
+  env, `lower_lambda`'s closure pair).
+- `crates/ail/src/main.rs`: `.arg("-lgc")` added to the clang
+  invocation in `build_to`.
+- `crates/ail/tests/snapshots/{hello,sum,list,max3,ws_main}.ll`:
+  mechanical s/@malloc/@GC_malloc/, 9 occurrences across 5
+  files. The IR is bit-identical to pre-14f modulo this
+  substitution — exactly Decision 9's promise.
+- `crates/ail/tests/e2e.rs`: new test
+  `gc_handles_recursive_list_construction` (+19 LOC).
+- `examples/gc_stress.{ailx,ail.json}`: new fixture.
+
+**Hash invariance verified.** Every existing fixture's def
+hashes are unchanged. The codegen and link line are downstream
+of canonical bytes; the AST schema didn't move; nothing on
+disk in `examples/*.ail.json` was touched. The new
+`gc_stress` module adds 4 new hashes, all unrelated.
+
+**Tests: 80/80 (was 79).** Existing 79 produce byte-identical
+stdout — only the allocator changed, semantics unchanged. New:
+`gc_handles_recursive_list_construction` builds a List<Int> of
+length 50 via recursive `Cons`, sums it (`1275`). Manual smoke:
+
+- `gc_stress.ail.json` → `1275`.
+- `list_map_poly` → `2 3 4` (unchanged).
+- `sort` → sorted list (unchanged).
+
+`cargo doc --no-deps`: 0 warnings (DESIGN.md item 6 invariant
+preserved through nine iters of feature work).
+
+**Pattern shape used in `gc_stress`** (caught a small typechecker
+constraint).  The first proposed shape `(case (lit-int 0) Nil)`
+doesn't parse — `pat-lit` takes the bare literal token, not a
+keyword-prefixed form, and `case` requires a pattern. Worked
+shape: comparison-and-bool-match, mirroring `sort.ail.json`'s
+`<=` arm:
+
+```
+(match (app == n 0)
+  (case (pat-lit true) (term-ctor List Nil))
+  (case (pat-wild) (term-ctor List Cons n (app build (app - n 1)))))
+```
+
+This is the canonical "if-then-else" pattern post-14d. Worth
+flagging for the stdlib brief: predicates that need to branch
+go through the `==` / `<` / `<=` builtin returning Bool, then
+match on that Bool with a wildcard fallback. Three lines for
+what `if` used to do in one — but uniform with the rest of the
+language, no special case.
+
+**GC integration notes.**
+- `GC_INIT()` is **not needed** on this build host (Arch
+  with `libgc 1.5.6`). libgc auto-inits via
+  `__attribute__((constructor))`.
+- No conservative-scan over-retention symptom observed: every
+  existing test's stdout byte-identical; behaviour preserved.
+- `-lgc` alone is sufficient for the link; pthread/dl come in
+  transitively from libgc.so's NEEDED entries.
+
+**Language is feature-complete enough for stdlib.** Iters 14d
+(redundancy removal), 14e (explicit tail calls), 14f (GC) are
+the three blockers identified at the 14b boundary. They are
+all done. Anything else (records as primitive, nested patterns,
+local rec let, type classes) layers on later without forcing
+stdlib rewrite.
+
+**Plan 15a.** First stdlib module: `examples/std/std_list.ailx`.
+Combinators: `length`, `append`, `reverse`, `map`, `filter`,
+`fold_left`, `fold_right`, `head`, `tail`, `is_empty`. Each
+combinator a fresh test vector for the parameterised-ADT +
+GC + tail-call combination. Authored in form (A) from day one;
+`.ail.json` produced via `ail parse`. Each combinator gets a
+dedicated e2e test.
+
+If 15a surfaces compiler bugs (likely — every prior dogfood
+iter has, see 14a's monomorphisation bug), debugger handles
+them inline. If a compiler limitation surfaces that genuinely
+blocks the stdlib (e.g. nested patterns turn out to be needed),
+that becomes its own iter before 15a continues.
+
+The architectural pin from Decision 6 governs: stdlib lives
+under `examples/std/` as `.ailx` source; tests load the
+generated `.ail.json`. `ailang-check` and `ailang-codegen`
+remain projection-agnostic.
+