extraction: remaining fixed-size TSNode walk stacks silently cap graph content #2388

Several AST walkers still keep their pending nodes in fixed-size stacks and stop pushing children once full. The graph then silently loses whatever lies beyond the cap, which our rule forbids: no work cap may decide graph content.

Remaining sites on main (approximate lines):

• internal/cbm/lsp/cs_lsp.c: 2680, 3102, 3279
• internal/cbm/lsp/php_lsp.c: 3259, 4146
• internal/cbm/extract_imports.c: 1423, 1532
• internal/cbm/extract_unified.c: 1802
• internal/cbm/helpers.c: 1016 (Nix)
• internal/cbm/lsp/ts_lsp.c: 4647 / 4693 / 4857 (being fixed in fix: grow the ts_lsp AST walk stacks so wide files keep every declaration #2091)

Fix pattern: cbm_walk_stack_reserve / cbm_walk_stack_release from #2375. The first chunk stays on the C stack and spills to the memory core for wide or deep trees. An allocation failure is logged (extract.walk_stack_alloc_failed) instead of silently truncating. #2373 did the same for the definitions walk.

Tests: each walker gets a wide or deep input that exceeds the old cap and asserts that all content is extracted: RED on main, GREEN with the fix.