release: lock semantic concurrency at hosted Pareto point

[justrach]

Summary

• preserve the production-measured semantic indexing default of 25 inputs × 4 concurrent batches
• add a release-gating regression assertion for the 100-input hosted wave
• bump aligned release metadata to 0.2.5848

The supported override remains capped at 8 for custom endpoints. This does not ship the experimental float16 transport.

Validation

• zig build test --summary all — 31/31 build steps succeeded
• zig build -Doptimize=ReleaseFast
• ./zig-out/bin/codedb --version — codedb 0.2.5848
• python3 scripts/e2e_mcp_test.py --binary zig-out/bin/codedb --project <release-worktree> — 76/76 passed

[github-actions]

👋 Thanks for the contribution! Quick heads-up: this repo lands changes on the current release/* branch, not main.

Please retarget this PR via Edit → base branch to the active release/* branch shown in the repository branch list.

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