feat(edge): compare API keys as fixed-width digests (#24)

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What

Remove the residual key-length timing signal from edge client authentication (issue #24).

authenticate already compares the presented key against every configured key with crypto/subtle.ConstantTimeCompare and no early exit, so response time does not leak which client matched or how far a guessed key got (ADR-0003). One residual signal remained: ConstantTimeCompare returns immediately when two byte slices differ in length, so the comparison cost still varied with the presented key's length relative to the configured keys'.

This PR reduces each configured key to its SHA-256 digest at construction (in set, so SIGHUP reload is covered too), and per request compares sha256(presented) against those 32-byte digests. Every comparison is now equal-width — this removes the length-dependent timing and the early-exit-on-length branch entirely. As a side benefit, the guard no longer retains raw key bytes past construction.

Acceptance criteria (issue #24)

• Authentication compares fixed-width digests; behavior (which identities authenticate) is unchanged.
• A brief note records that the length signal has been removed — in authenticate's doc comment and a Consequences bullet in ADR-0003.

Testing

• The full existing guard behavior suite stays green (auth, origin, reload, concurrency).
• New TestGuardAuthenticatesVariableLengthKeys pins the property the digest comparison must preserve: keys of very different lengths each resolve to their identity, and a wrong key sharing a real key's length is rejected. (Added and confirmed green against the pre-change code first, then carried through the change.)
• make verify: build, vet, -race tests, coverage 95.3% (gate 80%), golangci-lint v2.12.2 clean, govulncheck clean.

The public ClientKey type and the NewGuard/Reload signatures are unchanged, so the composition root needs no change.

Closes #24

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