perf: webview/app §8 measurements + migration-checksum heal

[danielss-dev]

Summary

Two things from one session: the webview/full-app performance pass (the PRD §8 targets the engine harness can't cover), and a migration-checksum data-persistence bug it surfaced and fixes.

Measured against the real production app via WebView2's DevTools Protocol (--remote-debugging-port) + a perf-gated window.__strand store hook. Win 11 / Ryzen 7 7700X, v0.7.0. Full numbers + methodology in docs/perf-baseline.md § "Webview / full-app baseline".

Performance results vs PRD §8

target	measured	verdict
Cold start < 1.0s	~407ms launch→shell paint, ~568ms launch→repo-interactive	✅ pass
Stage/unstage hunk < 50ms	~34ms isolated	✅ pass
Diff render 5,000-line < 100ms	~87ms normal diff; ~1460ms whole-file in non-virtualized Local Changes; bounded in Review	⚠️ mixed
Idle memory < 250MB	~280MB private / ~438MB WS (JS heap 7MB → WebView2 6-process baseline)	❌ over

Two follow-ups filed in TASKS.md: virtualize the Local Changes stacked diff pane, and reconcile the memory target with WebView2's floor.

ui/src/main.tsx gains a store hook exposed on window.__strand only when localStorage['strand:perf']==='1' (same gate as the perf instrumentation) — lets an external CDP harness drive the app without a native dialog. Zero effect on normal use.

Migration-checksum fix (data-persistence bug)

sqlx stores a SHA-384 checksum of each migration's SQL and refuses to open a DB whose stored checksum no longer matches the binary. Commit 3e1f0bb reindented migration 1's SQL (whitespace only), so every strand.db created before it — including public 0.x installs — failed to open, silently disabling session restore and all SQLite-backed settings persistence (the load error was caught and fell back to defaults).

• state::repair_migration_checksums recomputes each migration's checksum the way sqlx does and rewrites any stale _sqlx_migrations row, in Tauri setup() before the SQL plugin's migrator runs (the webview's first Database.load).
• Lossless: every migration is idempotent CREATE … IF NOT EXISTS, so the applied schema is identical regardless of whitespace — nothing re-runs and no user data (recents/icons/review state) is touched.
• sqlx + sha2 added as direct deps (already in the tree via tauri-plugin-sql).
• Process rule going forward: migrations are append-only; never edit an applied migration's SQL.

Verification

• cargo check, cargo clippy -p strand-tauri -- -D warnings, tsc -b (via tauri build).
• End-to-end CDP run of the perf measurements.
• Migration heal verified against the real broken DB: v1 checksum healed (5E02…→6D23…), v2 then applied, session restore + persistence work, no console error.

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