perf(code-index): resolve trait dispatch from relation keys

[ScriptedAlchemy]

Three receipt follow-ups, grouped because they share the relation-key code, the contracts retrieval surface, and one test file.

(a) Trait dispatch from relation keys

With MCP defaults (depth 3, trait dispatch on) the dispatch check read every direct callee's summary and incoming edges, on every page. augment_callee_dispatch_keys now finds every callee's container in one batched relation_keys step (incoming Contains), reads only the distinct containers to learn which are traits, and reads callees, implementors and their children only under a trait. The per-callee visit_trait_dispatch_targets is deleted.

relation_page_cost::a_default_callees_page_resolves_dispatch_without_reading_every_callee (105-callee fixture, defaults, page 1 and the cursor page), (point reads, adjacency queries, rows):

• master: FAILS (221, 108, 315): seed + 105 callee summaries + 105 edge reads + 10 hydrations, one fan-out per callee
• this PR: PASSES (11, 5, 315) on both pages, the same as with dispatch off

typed_callees_carry_their_read_cost… moves from (6, 3, 4) to (3, 4, 5).

Starship @ cc825b00, src/modules/mod.rs::handle, defaults, every page (built production,hotpath binary, isolated HOME, capped daemon):

	before (#2224)	after
reads / page	1,066	31
adjacency queries / page	315	12
whole walk (31 pages, 309 rows, same rows)	~33,000 reads	962 reads

page 1: items=10 total=309 reads=31 adjacency_queries=12 adjacency_rows=1550 bytes=81183 wall_us=87799
  stderr: tracedecay_cost: wall_us=87799 graph_sealed_reads=31 graph_staging_reads=0 adjacency_queries=12 adjacency_rows=1550 bytes_hydrated=81183
          tracedecay_metrics: before=30720 after=2647
defaults pages 31 rows 309 distinct 309 total reads 962
depth1 (no dispatch) pages 11 rows 107 distinct 107 total reads 118

(b) Callers (and the other symbol-graph reads) are metered

code_callers and the other symbol-graph primitive reads (symbol and signature search, implementations, type hierarchy) open their reader through the same CodeGraphReadCostMeter. SymbolGraphPortOutcome::{Completed, Partial} carry the receipt, and symbol_page puts it on the envelope.

typed_callers_carry_their_read_cost (callers of Walk::read): master FAILS with no cost ((Null, Null, Null)); now PASSES with {"graph_sealed": 3, "graph_staging": 0}, 1 query, 2 rows, plus a matching tracedecay_cost trailer. On starship, tracedecay_callers on handle: graph_sealed_reads=9 adjacency_queries=1 adjacency_rows=6.

(c) #2226: callable-code touched files

CodeQueryRow names the file each callable-code row was read from, CodeQueryPage::touched_files collects them, and the callable-code service puts them on the envelope, so these reads get the tracedecay_metrics footer.

callable_code_reads_carry_their_files_and_the_accounting_footer:

• tracedecay_code_symbol_search "Walk": ends with tracedecay_metrics: before=672 …, touched_files: ["src/walk.rs", "src/lib.rs"]. This already passed on master: symbol search is on the symbol-graph route, which reported files. It stays as the regression literal.
• tracedecay_callees (callable code): master FAILS, no footer; now PASSES, touched_files: ["src/walk.rs"], tracedecay_metrics: before=35 …. On starship, tracedecay_code_exact_occurrence "handle" now ends with tracedecay_metrics: before=4101 after=779.

Checks

• cargo test -p tracedecay --features test-transport --test mcp_suite: 570 passed, 1 failed. The failure is status_behavior_test::tracedecay_status_reports_the_sealed_branch_and_keeps_diagnostics_opt_in (status markdown now includes github_source since fix(configuration): provision the GitHub origin source binding #2221; red on master, not touched here). After the final rebase, -- graph_query test_map: 50 passed.
• tracedecay-code-index-runtime lib 515; tracedecay-application lib 476 + application_suite 64 + pr_tracking 7; tracedecay-contracts lib 419 + contracts_suite 276; tracedecay-query 263 + 77 integration; tracedecay-mcp 392 + 12 integration; tracedecay-code-index 257 + 166
• cargo clippy -p tracedecay-code-index-runtime -p tracedecay-application -p tracedecay-contracts -p tracedecay --all-targets --features tracedecay/test-transport -- -D warnings; cargo fmt --all -- --check; pnpm run contracts:check: contracts up to date

Fixes #2226

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-26T16:58:36.340708Z	200bfde	PR opened

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