perf(remote): release unviewed transcripts and trim live windows

[Juliusolsson05]

The phone previously accumulated every live transcript even on the session list. Transcript ownership now follows actual view subscriptions: unviewed JSONL/semantic bodies are not mapped or retained, and leaving the last view releases entries, tool indexes, mapper/fold state and identity sets. Selecting the session again backfills durable history and rejects stale pending responses.

Fixes #805
Refs #813

Acceptance criteria

• Several unviewed sessions retain zero transcript entries, tool indexes, seen UUIDs and semantic bodies; metadata remains available.
• Last-unsubscribe, reselect, rolled file identity, stale requests and session removal have behavioral coverage. Partial semantic suffixes wait for a fresh turn; committed content remains authoritative.
• Viewed live windows reuse the desktop pure count/estimated-byte trim planner (2,000 entries / 32 MiB triggers; 1,500 / 24 MiB targets), preserving semantic owners and tool pairs. Remote bookkeeping does not use desktop global registries.
• Indexes rebuild from retained entries in chronological order. Trimmed identities suppress old live replays but allow older pages to reload them in order.
• Exact history offsets survive trimming and all-duplicate pages; raw-record fan-out is not split. Paging/grace protects recently requested older content. Total entry count remains independent of the retained window.

Validation and reproducible evidence

• 137 local tests / 14 files pass: all remote transport/store tests plus shared entry/trim tests. Ten new retention regressions exercise real Claude, Codex and OpenCode mappers and semantic folds.
• The unviewed regression fails on parent fix(remote): bound outgoing queues and recover after overflow #813 with 4,096 retained entries. Fixed: three synthetic sessions each receiving 4,096 entries / 32 MiB logical ASCII text retain zero entries, indexes and UUIDs. These are collection/payload measurements, not production heap savings.
• A 2,100-entry live burst retains 1,500 entries with matching indexes; 300 entries of 128 KiB each trigger byte trimming below the count threshold. Tests verify replay suppression, chronological reload, tool pairs, semantic owners and exact cursor offsets.
• Typecheck, test contract, client production build and diff check pass. Full applicable repository checks passed against the updated main base. Existing bundle size/mixed import warnings remain.

Limits and tradeoffs

Safety constraints (active/history semantic ownership, tool pairs, missing stable identity/anchor and whole raw records) can pin a viewed window above its nominal target. Older-history reading has a 30-second trim grace. Small UUID-only tombstone sets grow during a continuously viewed session to preserve exact replay deduplication; all are released when that view ends. Live wire entries lack byte offsets and retain provider-marker fallback; history pages use exact offsets when supplied. Re-selection costs a backfill and an interrupted live turn may wait for durable completion. Mobile heap/FPS gains remain unmeasured.

Plan committed before code: docs/superpowers/plans/2026-09-05-remote-transcript-retention.md.

Dependency #813 is merged. This PR now targets main bb84772a; range-diff confirms its implementation patches are unchanged after rebase. Current head: c4f175f0. Fresh CI passed before merge. The user authorized merging once current checks pass on 2026-09-07. The parser source alias remains included; the clean-build failure was reproduced and corrected against an unbuilt parser checkout.

Merged with explicit user authorization on 2026-09-07 as 13258ffa after both current-head CI gates passed. No outstanding review feedback. The resolving issue is closed.