trinity cards: own the brain region test targets that gHashTag/trinity#1333 restores, and drop src/brain from research.unreferenced-sources #5953

Problem

gHashTag/trinity#1333 (Closes gHashTag/trinity#1331) gives build.zig real build steps for the brain regions again. The S01 checker (tools/trinity_manifest.py, vendored byte for byte into trinity as external/t27/) finds 11 targets that no card owns:

• step:test-basal-ganglia, step:test-reticular-formation, step:test-locus-coeruleus, step:test-brain, step:test-brain-stress
• test:src/brain/basal_ganglia.zig, test:src/brain/reticular_formation.zig, test:src/brain/locus_coeruleus.zig, test:src/brain/brain.zig, test:src/brain/integration_test.zig, test:src/brain/stress_test.zig

Trinity's capability index (tools/capabilities.py) fails on each of these as UNASSIGNED_TARGET. That makes the "Headless profile from a clean clone" job (reproduce.yml) red on #1333. The cards belong to t27, so this cannot be fixed in trinity without a second home for the truth.

research.unreferenced-sources also names trinity:src/brain as a directory that no file named by build.zig reaches. Once #1333 lands, build.zig names six files under src/brain as test roots, so that line becomes false.

Scope

1. Add a capability card under specs/trinity/capabilities/ that owns exactly those 11 targets. Its disposition, profile, acceptance and evidence must each be true. Its evidence may say measured only if a public CI run measured it, with the command and commit recorded (docs/system/project.md).
2. Remove src/brain from the title and IMPLEMENTATION of research.unreferenced-sources.
3. Add seals and a regenerated conformance/trinity/report.json.

The order problem

The cards are held to the inventory of the pinned tree gHashTag/trinity@976df5174, and none of the 11 targets exist there. At the pin, src/brain has 46 .zig files: 3 of them are reachable as modules, and the build defines no brain test or step. So trinity_manifest.py check against the pinned inventory reports UNKNOWN_TARGET for each of the card's 11 patterns. No t27 workflow runs that check. The regenerated report.json must keep those findings and say why; they must not be hidden or tolerated in code. Re-pinning S01 is separate work and out of scope here.

Acceptance

• python3 tools/trinity_manifest.py --self-check passes.
• Checking an inventory of gHashTag/trinity at the head of gen-zig and gen-c: ExprCast falls through default arm (silent drop / '/* unsupported */') #1333 reports none of the 11 UNASSIGNED_TARGET findings, and no finding at all for the new card.
• Seals for both cards verify (t27c seal --verify), and seal coverage and seal currency pass.
• t27 CI is green.