fix(extract): AST walker stacks grow instead of silently capping content - #2375

Follow-up to the review of DeusData#2091, on top of the contributor's change.

The three ts_lsp.c walks (infer_return_type_from_body,
rebuild_signatures_from_ast, convert_signature_type_params) now keep
their first 256 slots on the C stack, as the old fixed arrays did, and
grow on the heap through cbm_walk_stack_reserve / cbm_walk_stack_release
(DeusData#2375) instead of cutting TSNodeStack storage from ctx->arena:

- Scratch lifetime: ctx->arena in the per-file pass is the result
  arena, kept until the result cache is freed, and the return walk runs
  once per untyped top-level function. The heap chunk is now released
  when each walk ends.
- No silent drop: when the stack cannot grow, cbm_walk_stack_reserve
  logs extract.walk_stack_alloc_failed and the walk stops. The return
  walk then answers "unknown"; the signature walks leave the
  declarations not yet reached as extract_defs registered them.
- Traversal order: children are filled in cursor order (still one O(N)
  pass) and popped last-child first, exactly as main's walks did, so of
  several returns the later one is still tried first.
  tslsp_return_walk_order_prefers_later_return pins it; it fails under
  a first-child-first walk.

The TSNodeStack arena initializer in extract_node_stack.h is no longer
needed and is dropped, so that header is unchanged from main. The two
stack-cap tests now fail instead of passing when their buffer
allocation fails.

Co-authored-by: Joshua Richter <[email protected]>
Signed-off-by: Martin Vogel <[email protected]>

walk_yaml_mapping (dotted key paths, 256 frames) and
scan_yaml_for_infra_bindings (512 nodes) kept their pending nodes in
fixed-size stacks and stopped pushing once full. A mapping with more than
256 nested sibling mappings lost the full key_path of every leaf past the
cap: `k299.url` was only recorded as the shorter path that the walk of a
nested block emits on its own.

The infra Route pass decides on those paths. A `data:` payload block with
300 entries lost `data.svcN.webhook_url` for N >= 256, the payload deny
never saw those values, and 44 of them were minted as infra Route nodes.

Both walkers now start on the same-sized on-stack chunk and grow through
cbm_walk_stack_reserve / cbm_walk_stack_release (#2375). If the stack
cannot grow, the failure is logged as extract.walk_stack_alloc_failed and
the walk stops, instead of truncating silently.

Part of #2388.

Signed-off-by: Gaurav Dubey <[email protected]>

elixir_compute_complexity kept its pending frames in one fixed 4096-entry
array (48 bytes each, 196,608 bytes in a single stack frame) and stopped
pushing children once it was full. Children are pushed last to first, so a
wider body lost its FIRST children and the function reported a lower
complexity with no signal, on exactly the largest functions. Worker threads
do not get the 8 MB main-thread stack, so the array was a risk of its own.

It now uses the walker pattern cbm_compute_complexity has used since DeusData#2375:
a 512-frame on-stack first chunk that spills to the heap through
cbm_walk_stack_reserve (memory core, class EXTRACT) and is released with
cbm_walk_stack_release. If the stack cannot grow, every metric reports
CBM_WALK_METRIC_UNAVAILABLE instead of an undercount. This answers items 2
and 3 of the 2026-08-20 review.

The new test builds a 5000-statement body of alternating if and for. The
fixed stack reports complexity 4095 for it instead of 5000.

Signed-off-by: Martin Vogel <[email protected]>