chore: fix pre-existing main breakages blocking clean overlay composition (#821)

[aarontrowbridge]

Closes #821. Fixes the two pre-existing breakages that red every overlay composition check on pristine origin/main, verified on a fresh origin/main (6afdb9d) worktree and materialized tree before and after.

Breakage 1 — server-session.ts(497) diff_version type clash

Repro: materialize (canonical v1.18.29 + overlay) → tsgo -b in packages/app:
error TS2741: Property 'diff_version' is missing ... but required in type 'SessionCache'.

Root cause: canonical's server-session.ts builds its store literal without diff_version (v1.18.29 and current upstream HEAD) and passes that draft to dropSessionCaches, which the overlay types with diff_version required. The field is an overlay-only cache-invalidation counter — there is no canonical wire shape to match — so the reconciliation widens the parameter type: diff_version becomes optional on SessionCache and the delete is guarded. The overlay's own global store type (global-sync/types.ts State) still declares it required; only dropSessionCaches' parameter widens.

Verification: the materialized typecheck error set on the fixed branch is byte-identical to the base set minus exactly this one error (19 → 18; the remaining 18 are the known overlay-vs-v1.18.29 API drift on surfaces PR #819 re-homes — out of scope here, no drive-bys).

Breakage 2 — i18n parity failure

Repro: bun test packages/app/src/i18n/parity.test.ts in the materialized tree fails with missing: ["session.exportTrace"] (en.ts has the key — added at the fork pin — none of the 17 non-English app locale dicts do; the ui and desktop domains are clean).

On the issue's 'am' attribution: it traces to PR #819's review notes and does not reproduce — the overlay carries no am dict and its parity test checks the 17 locales it owns; the materialized tree's only parity failure is this missing key across all 17. Restored the key in all 17, following each table's own terminology (br is pt-BR upstream). Inventing overlay am dicts was rejected deliberately: in the composed tree they would overwrite canonical's complete Amharic tables with a smaller fork-era keyset — the opposite of restoring keys.

Also (third pre-existing main breakage, surfaced by the repo's own tooling)

refresh_manifest.mjs / drift_gate.mjs revealed that the committed manifest lags the overlay by the three #774 session-ui files (diff-gutter-extension.ts, editable-diff-view-core.ts, editable-diff-view.tsx — in overlay/, absent from manifest.files), so the drift gate fails on pristine main with "stray files". Commit 1 adopts them via the canonical repair path (refresh_manifest.mjs against the v1.18.29 upstream tree). Flagging it here since it's adjacent to #821's scope but not named by it — happy to split it out if reviewers prefer.

Commits (each green, drift gate PASS at each)

Commit	Behavior
948ca66	manifest refresh — adopt the three #774 session-ui overlay files
dc08246	reconcile SessionCache with canonical server-session shape (optional diff_version)
872a521	restore session.exportTrace in the 17 non-English app locales

Verification summary

• Drift gate: PASS at every commit (579 files, overlay and manifest in sync).
• Materialized tree (fresh, from this branch): server-session.ts(497) error gone; parity test 4/4 pass.
• global-sync unit tests: 37 pass, unchanged from base (the 2 bun-test Solid-server module errors are identical on pristine main — environment artifacts, not touched).
• No collision with sibling PR Re-home the session & device lifecycle base contract onto the app overlay (canonical v1.18.29) #819 (it touches session-list-state/snapshot code on its own branch; this branch touches session-cache.ts, i18n tables, manifest.json only).

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