fix(state): never drop manual flags — non-destructive sync, scoped accept-edits

[mhenrixon]

Problem

In consuming-app PRs (e.g. getzazu/app#2918), locallingo churned state unrelated to the PR: manual: true removed from hand-curated keys, added to unrelated namespaces, inconsistently across PRs.

This was not a version regression — the core files were unchanged since the bin/translate extraction (b007b34). Three extraction-era flaws interacted:

1. lingo sync destroyed state. It replaced every target entry with {source_hash} only, wiping target_hash + manual: true app-wide. The pre-extraction bin/translate sync preserved both (entry["manual"] = true if locale_state.dig(key, "manual")); the extraction dropped that line. zazu auto-runs bin/lingo sync from a Claude Code Stop hook whenever locales change, so the wipe fired silently in unrelated PRs.
2. translate --force-key dropped manual when rewriting the entry — and the outdated validator's suggestion text told agents to run exactly that command.
3. accept-edits blanket-stamped manual: true on every key of every namespace, and the manual-edits validator suggested that blanket command for a single drifted key.

Which way flags flipped depended on which command ran last on a branch — hence the "random" churn.

Fix

• sync_state! merges: refreshes source_hash, preserves target_hash/manual. Hand-edit drift stays visible until explicitly accepted (deliberate divergence from the old script, which silently absorbed edits unprotected).
• update_locale_state carries manual: true through retranslation. No command removes the flag anymore — unprotecting requires editing the state JSON by hand.
• accept-edits scoped: unscoped accepts exactly the keys the manual_edits validator flags; --key KEY (repeatable) for surgical accepts; --all for the old blanket (initial adoption).
• StateStore#save skips byte-identical files — unrelated namespace files are never rewritten.
• Validator suggestions are key-scoped and safe to follow verbatim; outdated on a manual key now says hand-update + re-accept instead of pushing machine translation over curated text.
• Docs (commands, drift-state, validators), README, CHANGELOG updated. No version bump — rake release[x.y.z] owns it.

Verification

• 98 examples, 0 failures (13 new specs covering sync preservation, force-key flag survival, the accept-edits matrix, suggestion texts, and byte-identical unrelated files); RuboCop clean on changed files.
• E2E against real zazu state (Gemfile path override, restored after):
  • bin/lingo sync on a clean tree: zero fr/af churn (previously stripped thousands of manual/target_hash lines); only legitimate additions of previously untracked en hashes.
  • Hand-edit one fr value → validate --strict flags it with the scoped suggestion → running that suggestion verbatim changed exactly one entry in one file, everything else byte-identical.

Follow-up in getzazu/app (after release)

Bump the gem, update agent docs referencing blanket accept-edits to the --key form, and optionally reconsider the Stop hook (sync, even fixed, masks outdated detection for en-only edits made without translating in the same turn).