fix(php): index trait methods by receiver while flattening (#1265)

[DeusData]

What

flatten_trait_into_class (PHP LSP) found each trait's methods by scanning every function in the registry. Every flattened method adds more functions, so the scan grows with its own output. A file with many trait-using classes paid O(classes × functions) (#1265).

The class-field pass now keeps a receiver -> function-id index, updated incrementally as functions are registered. Flattening reads only the trait's own entries that were registered before the current class:

• Same output: it visits only entries registered before flattening started, the ones the old scan accepted, so the result is identical.
• Fallback: if the index cannot be built, the previous full scan runs unchanged.
• Memory: the index and its per-receiver lists live in the LSP call's arena.

Evidence

• Graph identity, from a clean build of this branch (7dc07d9 on main 4527fc7, build log from scratch) against a clean build of main c06465b, parallel:

  repo	node dump	edge dump
  laravel/framework	identical (sha256 c43f263775f36cd5)	identical (3de7805591d92837)
  filamentphp/filament	identical (87991e1975daf8ce)	identical (fadf65d161b0ec65)
  grokability/snipe-it	identical (50ee2e8cfcc57c56)	identical (d22bb4d5270670ed)

  All three are trait-heavy. The only commit between c06465b and 4527fc7 is fix(php): index the single-file LSP type registry #2702, which also preserves output, so the comparison covers both changes together. An earlier combined build was also byte-identical on every PHP and C# corpus repo, parallel and sequential. That build held this change, fix(php): index the single-file LSP type registry #2702 and the iterative C# walker.

• Work counter (CBM_LSP_WORK, deterministic, no clock), using the test-only work ceiling from fix(php): index the single-file LSP type registry #2702:

  • phplsp_trait_flatten_work_is_linear_issue1265, 100 -> 200 trait-using classes, gate: ratio ≤ 2.5.
    • With the index disabled (full scan): work 61,344 -> 182,252, ratio 2.97, RED.
    • With the index: work 1,958 -> 3,766, ratio 1.92, GREEN.
  • phplsp_trait_collision_shapes_bounded_work_issue765: trait collision shapes (PHP: trait whose name collides with an aliased use … as import it composes → unbounded recursion in definitions pass, OOM (SIGKILL) #765) stay bounded. Work is 1,225 without the index and 343 with it. Main does not reproduce PHP: trait whose name collides with an aliased use … as import it composes → unbounded recursion in definitions pass, OOM (SIGKILL) #765, so this test pins the behaviour instead of fixing a red case.

Suites on 7dc07d9 (ASan test-runner): php_lsp, parallel, pipeline, complexity and incremental, 941 passed.

Lint: internal/cbm/lsp/ is outside CI's LINT_SRCS / LINT_HDRS, so I ran the gates directly on the changed files.

• clang-format is clean on the lines this PR adds. php_lsp.h already differs from clang-format on main, outside this diff.
• cppcheck 2.20 first flagged autoVariables: the pass stored the address of a stack-local index in the context. The pointer was restored before return, but the second commit moves the index into the arena, and the warning is gone. One existing style note remains (redundantInitialization of qualified in php_lsp.c), outside this diff.
• lint-memory-core and the NOLINT check are clean.

Head e9b334e merges main 27c3303. Main has not touched php_lsp.c, php_lsp.h or tests/test_php_lsp.c since 4527fc7, and merge-tree is clean.

Fixes #1265. Refs #765.