feat(core): add first-class Pi session indexing (#23)

Pi cannot be read as another linear JSONL stream. Its history is a tree with
a durable leaf, orphan roots, branch summaries, and two compaction forms, so
the active context is something the format states rather than something line
order implies. The adapter keeps those semantics inside itself and projects
the result into the existing canonical tables.

Sessions are keyed by (normalized header cwd, header id) rather than by path,
because Pi's --session-id lookup is project-local: two projects may reuse an
id, while a move or an identical copy is still one session. Discovery covers
both layouts Pi writes and fingerprints each file by mtime, ctime, size and
inode, so a rewrite that preserves mtime is not read as unchanged.

Abandoned branches are preserved rather than dropped. Visibility becomes
three-state -- visible, inactive, hidden -- and helpers return only visible
rows until includeInactive asks for the superseded path, labeling every row
so a caller knows which it holds. Usage counts all three, because an
abandoned call still spent tokens; message_count reports only the visible
transcript.

A committed MIT-licensed oracle transcribed from Pi 0.83.0 pins the context
algorithms, and a fixed-seed differential runs 512 generated sessions against
it on every test run. Schema changes are additive.

docs/adr/0001-parse-core-and-persist-layers.md

@@ -21,19 +21,36 @@ binding-agnostic and does not need a per-binding implementation.
 **Decision.** Split indexing along two orthogonal axes.
 
 - **Provider axis — a registry of pure adapters.** Each source (Claude Code,
-  Codex, Kimi Code, later Pi, …) is a provider adapter implementing one complete
+  Codex, Kimi Code, Pi, …) is a provider adapter implementing one complete
   boundary: serializable descriptor metadata, `watchRoots(root)`,
-  `discover(context) → IndexUnit[]`, `parse(unit, cursor) → Iterable<Record>`,
+  `discover(context) → IndexUnit[]`,
+  `parse(unit, cursor) → Iterable<Record>`,
   and `raw(lookup)`. An `IndexUnit` is deliberately not a file abstraction: Kimi
   uses one session directory containing state plus multiple agent wire logs. An
   adapter is *pure*: it emits normalized records and never touches a database.
+  Discovery receives a read-only view of the provider's already-indexed session
+  paths. A full-reparse adapter can therefore attach `retractSessionIds` to a
+  replacement or tombstone unit without querying SQLite itself. Retraction and
+  replacement records commit in the same unit transaction, so a failed parse
+  preserves the last complete snapshot.
+  Provider identity may therefore be richer than a wire-level ID. Pi, whose
+  explicit session IDs are project-local, deterministically namespaces the
+  header ID by the normalized header cwd; source paths remain provenance rather
+  than identity, so copying or moving a transcript does not rename the session.
   Adding a source means adding one adapter and registering it; nothing else
   changes. `parse` exposes an iterator as its common interface and streams when
   the provider semantics permit it. An adapter may buffer one complete
   `IndexUnit` when correctness requires whole-unit semantics — for example,
-  Codex duplicate reconciliation or Kimi `context.undo` / `context.clear`
-  replay. Each adapter maps its own resume/change semantics onto the existing
-  `mtime` and `lines_processed` cursor pair in `index_state`. The emitted
+  Codex duplicate reconciliation, Kimi `context.undo` / `context.clear`
+  replay, or Pi tree projection. Each adapter maps its own resume/change
+  semantics onto an opaque cursor stored exactly in `index_state.cursor`;
+  `mtime` and `lines_processed` remain compatibility/index-inspection columns.
+  A provider-wide replay is a destructive snapshot boundary. Discovery reports
+  any source location it could not enumerate; destructive rebuilds fail closed
+  when such a report exists. Cleanup, parsing, persistence, FTS rebuild and
+  marker publication commit in one transaction, so a parse failure cannot
+  replace the last-good index.
+  The emitted
   `TranscriptRecord` stream is also the input to provider-independent session
   detail assembly; see ADR-0007.
 - **Persist axis — one shared orchestration.** A single provider-agnostic,
@@ -60,9 +77,11 @@ is disentangling the currently interleaved parse-and-write inside `indexJsonl` /
 
 The normalized `TranscriptRecord` union is the stable center of the design, and
 SQLite is one serialization adapter for it. Provider-only concepts are either
-projected lossily into that language or ignored. The registry,
-not provider switches, drives both indexers, watcher roots, persisted source
-roots, source catalog/UI labels and colors, and raw-record routing. Adding Pi
-therefore changes the Pi adapter, its registration, and its conformance tests;
-the shared schema, persist layer, indexers, settings, query API, and renderer do
-not acquire Pi-specific branches.
+projected lossily into that language or ignored. A genuinely shared concept may
+extend the canonical language by explicit decision: summary-generating model
+calls, for example, carry the same normalized input/output usage as ordinary
+messages. The registry, not provider switches, drives both indexers, watcher
+roots, persisted source roots, source catalog/UI labels and colors, and
+raw-record routing. Adding Pi therefore adds no Pi-specific branch to the
+shared schema, persist layer, indexers, settings, query API, or renderer; the
+provenance and summary-usage additions are provider-neutral contracts.