fix(session): bound in-memory session mirror

[code-yeongyu]

Summary

• Bound the ResidentStringStore to a conservative 64 MiB FIFO cache; the JSONL remains authoritative when a blob is evicted.
• Trim superseded pre-compaction message entries from the live SessionManager mirror while preserving metadata and reloading pruned entries from disk for branching.
• Added regression coverage for resident bytes, compaction trimming, and disk-backed branch reload.

TDD evidence

RED (commit 4c7cc71cc, before implementation):

Test Files  1 failed (1)
Tests       3 failed (3)
expected 73400520 to be less than or equal to 67108864
expected ... to have a length of 3 but got 4

GREEN (commit 91d6a12b1 plus final fixture commit, current HEAD):

Test Files  14 passed (14)
Tests       142 passed (142)

The literal requested filter src/core/session-manager was also attempted, but this repository's Vitest roots contain tests under test/; that invocation exits 1 with No test files found. The valid remote filters used above are test/session-manager/session-mirror-budget.test.ts and test/session-manager.

Verification

• Remote Mac runner: MAC_TEST_TIMEOUT_S=2400 bun /tmp/senpi-mac-test.mjs fix/mem-session-mirror-budget -- test/session-manager (exit 0).
• Rebased onto current origin/main (b618e8cd4) before final push.

---

Summary by cubic

Bounds the in-memory session mirror so long-lived sessions no longer consume unbounded RAM; the JSONL stays authoritative and evicted or trimmed data reloads from disk when read.

• ResidentStringStore now evicts the oldest externalized strings past a 64 MiB per-session budget; evicted blobs reload from the JSONL when read.
• Compaction trims superseded pre-compaction message entries from the live mirror while preserving metadata; branching or reading a pruned entry reloads it from disk instead of throwing.
• Full-history and compact context reads cache separately; the compact cache holds externalized references so it stays bounded, and missing compact entries are recovered in one batched disk read and stay re-evictable.
• Adds regression tests covering the byte budget, compaction trimming, cache behavior, and disk-backed branch reload.

^{Written for commit 2a99856. Summary will update on new commits.}