fix(app-bundle): guard tool-count-label split() against undefined translations (#987)

[aarontrowbridge]

Fixes #987 — the production hub-panel crash Aaron hit live during #955's cutover smoke.

What

TypeError: Cannot read properties of undefined (reading 'startsWith') on cold page load, recurring and restart-masked. Mechanism: the base session-ui/src/components/tool-count-label.tsx renders via split(i18n.t(...)) — with the i18n dict not yet loaded (async resource) or a key missing, i18n.t(...) returns undefined; split(undefined) produced {before: "", after: undefined} and the memos' one().after.startsWith(...) threw inside a Solid effect.

The fix (overlay twin)

• overlay/packages/session-ui/src/components/tool-count-label.tsx — the base file plus a guard: split(text: string | undefined) returns {before: "", after: ""} for falsy text (honest empty render until the translation exists, the language.tsx fallback discipline), with a header comment naming the bug and mechanism.
• Overlay mechanism verified (the step-3 check): materialize.mjs walks the entire overlay/ tree recursively and applies every package — session-ui is a first-class overlay package (25A/29M→30M in the manifest). The twin materializes byte-identical into the composed tree.
• Manifest re-registered via refresh_manifest.mjs (620 files); drift gate PASS.

Tests

packages/extension/test/tool_count_label_guard_987.test.ts (vitest, the #964 contract/source pattern — the app has no render harness):

• the twin's actual split/common code is extracted from the file, transpiled, and exercised — not a re-typed copy;
• the memo derivations that threw in production run against a missing key (t → undefined) across all six plural categories — no throw, empty render;
• the unguarded base file reproduces the exact production crash under the same harness (verified), the twin does not;
• manifest registration asserted at the exact hash (drift-gate invariant).

Verification

• pnpm run typecheck green (schema, amico-run, extension).
• Extension vitest suite: my branch vs a clean origin/main baseline, both with the suite's overlay-mutating overlay_sync.test.ts benched (see hazard note): identical pre-existing failure set of 17 tests / 9 files (auth/bootstrap/service/terminal/cli_gate — environment-dependent: local OPENCODE_DB, fork checkout presence). Zero new failures; overlay_known_fixes_964, editable_diffs_wiring, packaging, yellow_chunks_853 all pass.
• ⚠️ Pre-existing hazard (live): overlay_sync.test.ts still runs overlay-sync.mjs --apply against the local fork checkout and mutates ~260 committed overlay files mid-suite (the chore(test): overlay-sync test applies to the local fork checkout's committed tree — point it at a temp tree #842 hazard — it clobbered this branch's twin twice during baseline runs; Sync-reconciliation: the fork→overlay sync clobbers amicode-side fixes (#929's fix regressed by ff7b69c8; the diff_version fix owes a fork mirror) #964's guard fails after any such run). Follow-ups: restore the chore(test): overlay-sync test applies to the local fork checkout's committed tree — point it at a temp tree #842 temp-tree fix, and mirror this fix to the fork's local/amicode branch per Sync-reconciliation: the fork→overlay sync clobbers amicode-side fixes (#929's fix regressed by ff7b69c8; the diff_version fix owes a fork mirror) #964 so the next fork→overlay sync brings the fix, not the regression.

Refs #955, #964, #842.

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