chore(release): v0.4.0

[tobert]

The mechanical half of the release ritual, so the tag is the only thing left to do by
hand. version → 0.4.0 in Cargo.toml and the lockfile, the unreleased section retitled,
a fresh empty one opened above it.

0.4.0 rather than 0.3.1 was your call on 2026-08-16, and the section earns it: three
backend kinds arrived (bfl, gemini-images, dashscope), write_cas and kaibo cas write are new surfaces, telemetry gained the metrics signal, and backends stopped seeding
their own key sources. A minor, not a patch.

⚠️ One thing to check before tagging

The date reads 2026-09-03 and it is a guess — you said you'd likely pick this up in
the morning. It must match the day the tag is pushed:

sed -i 's/^## \[0.4.0\] — .*/## [0.4.0] — YYYY-MM-DD/' CHANGELOG.md

It is the one thing in this PR a later reader cannot infer or correct from context.

Pre-tag gates — run, not assumed

Every item in AGENTS.md's Cutting a release, checked in the session that produced this
branch:

gate	result
kaish pin current and no known unfixed bug	0.17.1 — the readlink -f break that held this release is fixed and measured
turso pin current	=0.7.1 holds; 0.8.0-pre.7 is a prerelease
cargo tree -i empty	aws-lc-rs, aws-lc-sys, mimalloc, openssl-sys — all absent on the musl target, with ring as the control that the query works
musl binary static	statically linked, stripped / not a dynamic executable, runs and reports its version
sandbox probes re-run	A–G against both kaish pins and diffed — required by the kernel bump

The static assertion was also run against a dynamic binary to confirm it discriminates,
because a check nobody has seen fail is a check nobody has tested.

Suite 1329 passed / 0 failed. kaibo --version reports kaibo 0.4.0.

After the tag

.github/workflows/release.yml builds the matrix on v*. Then the ritual's last steps,
both deliberately manual:

1. Verify a fresh asset the way a user would — gh attestation verify, and cosign verify-blob with the new tag's identity. Note the floor is now cosign ≥ 2.5,
   measured this session (2.4.0 genuinely cannot read the bundle).
2. scripts/bump-tap.sh v0.4.0 to point the Homebrew tap at the new release.

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