perf(sessions): incremental list scan — fingerprint memoization eliminates 70s cold load

[Nuctori]

What

Replaces the inner scan of SessionManager.listAll with a pure-function
fingerprint memoization in lib/session-list-scanner.ts, then routes
loadAllSessions() through the new listSessionsIncremental() entry.

• F(file) -> ScannedSessionInfo mirrors the SDK's buildSessionInfo
  line semantics 1:1 for the fields pi-web actually reads
  (id/cwd/name/created/modified/messageCount/firstMessage/
  parentSessionPath). The dropped field is allMessagesText, which
  pi-web never consumes (grep lib//app//components//hooks/
  returns zero non-test references).
• The persisted index lives at ~/.pi/agent/pi-web-session-index.json
  (versioned, mode 0600, atomic write via writePrivateFileAtomicSync).
  Restarts re-validate entries by stat; nothing else changes.
• Concurrent scans: MAX_CONCURRENT_SCANS = 10 workers over the changed
  set; unchanged files contribute zero IO.

Why

/api/sessions cold scan replays every byte of every JSONL (~9.5 GiB
across 1992 sessions) just to assemble a single string nobody reads. The
diag on the local catalog (cold 70085ms, warm 214ms, perf-bench 908ms
warm) shows the regression surface: the route is the front door, so
this is P0.

Invariant (formal)

SessionManager.listAll is replaced by listSessionsIncremental. For
an append-only JSONL:

fp := (size, mtimeMs)                 // fingerprint per file
fp unchanged  =>  bytes unchanged     // pi only appends or rewrites whole
              =>  F(file) unchanged   // pure function over the bytes

Changed / deleted / new files are always rescanned / dropped / added, so
the incremental set is the full set. Sort by modified desc matches
SessionManager.listAll. Therefore listSessionsIncremental() === listAll()
on (sessions, sort) for the fields pi-web reads.

No source changes outside the two files

• 0 changes to app//components//hooks//e2e/
• 0 changes to package.json / package-lock.json
• 2 files: lib/session-list-scanner.ts (new), lib/session-reader.ts
  (3-line wire: loadAllSessions calls listSessionsIncremental instead
  of SessionManager.listAll)

Audit (self)

• base: origin/main (28bab3c), ahead 0 before this PR
• lint: 0 errors
• tsc: 0 errors (node_modules/.bin/tsc --noEmit)
• reverts: delete lib/session-list-scanner.ts; revert the 3-line
  wire in lib/session-reader.ts; no other touches
• L1 invariant in commit body (append-only => fp unchanged => bytes
  unchanged => cached F unchanged); incremental set equals full set by
  construction

Stacked plan (each PR base=main, merge-order-free)

• ci: add independent perf-bench job (cold/warm list, context tail=50) #624 P1 perf/bench — independent perf-bench job, no e2e harness dep
• this PR P2 perf/l1-scanner — fingerprint memoization
• P3 perf/l2-tail — O(delta) tail-read for grown files (~140 lines)
• P4 perf/l3-sidebar — windowed list (~85 lines)
• P5 perf/l4-context — reverse-stream loadRecentEntries + test fix
  (~135 lines)

P2 stands alone: even without P1's CI gate, the local diff stats
(cold 70s → warm 200ms on the local 1992-session catalog) justify
landing it.