During launch, the bootstrap-wait loop polls each member and, per member, re-read
and re-JSON.parsed the same growing transcript tail (readRecentBootstrapTranscriptOutcome
was the top main-thread JS hotspot at ~21% during bootstrap, ~40% with its helpers).
The same file was parsed once per member per poll.
Memoize the parsed tail by (filePath, mtime, size) in a shared cache so the file is
read + parsed once per change and reused across all members. The per-member filter
and failure/success scan is byte-for-byte the same logic; only the redundant read +
JSON.parse is removed. Cache is bounded (LRU, same cap as the outcome cache) and
invalidated on mtime/size change, matching the existing outcome cache semantics.
Adds a test asserting the tail is parsed once and shared while per-member outcome
detection is unchanged.
35b76f1354