ci: gate b0_alloc_connector allocations (design 054 §5)

[lxsaah]

Stage 1 of the 054 implementation plan. It makes b0_alloc_connector a CI gate before any connector-boundary code changes, so every later stage is measured against a gate that already works.

Changes

• Makefile: new bench-gate target that runs cargo bench --package aimdb-bench --bench b0_alloc_connector. It is added to .PHONY and to the help text.
• .github/workflows/ci.yml: new Bench Gate job (same checkout and toolchain setup as makefile-build) that runs make bench-gate. comprehensive-check now needs it.

No bench code changes. EXPECTED and the committed baseline stay as they are.

Verification

• make bench-gate passes locally, with all 8 rows at their EXPECTED values.
• With inbound_route changed from 0 to 1 in EXPECTED, the target fails ("inbound_route: expected 1, measured 0.00") and exits non-zero. The change was reverted.
• The bench run itself takes seconds; compiling aimdb-bench took about 1 minute locally.

After merge

• Mark Bench Gate as a required check in branch protection.

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