fix(php): index the single-file LSP type registry - #2702

What

cbm_run_php_lsp (the single-file PHP LSP pass) never finalized its type registry. Every lookup during single-file resolution therefore scanned the stdlib plus every definition of the file linearly: O(lookups × definitions), quadratic in the size of one file. The cross-file path (cbm_run_php_lsp_cross) already indexes its registry.

The registry is now finalized with first-registered semantics (index_first_registered), and re-indexed once after trait flattening registers more methods:

• First-registered semantics: the first registration of a duplicate QN wins, which gives exactly the answers of the old linear scan.
• Re-index after flattening: this keeps lookups from tail-scanning the methods that flattening adds.
• Memory: the index lives in a scratch arena that dies with the call, as in the cross-file path.

A test-only work ceiling (CBM_LSP_WORK_CEILING_HIT, compiled in seam builds only) lets the work-counter test stop a reverted, unbounded loop deterministically instead of hanging the runner. Product builds have no ceiling, so it can never decide graph content.

Evidence

• Graph identity: byte-identical to main on every PHP and C# corpus repo, parallel and sequential, including dotnet/roslyn and dotnet/runtime. This was proven on the combined identity build L6-y-fbd9a2ae (this fix + the trait-method index + the iterative C# walker), all of which preserve output. This PR's code is unchanged from that build.
• Work counter (CBM_LSP_WORK, deterministic, no clock), phplsp_single_file_lookup_work_is_linear_f12, 1,000 → 2,000 chained classes (gate: ratio ≤ 2.5):
  • without the fix: work 789,210 -> 2,578,710 (ratio 3.27), RED;
  • with it: work 3,208 -> 6,289 (ratio 1.96), GREEN.

Suites run locally: php_lsp, parallel, pipeline, complexity, incremental: 927 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.
• cppcheck 2.20 reports one style note in php_lsp.c (redundantInitialization of qualified, about line 4311). It is in existing code outside this diff, so I left it untouched.