Stabilize streams memory and read performance

[sorenbs]

Summary

• stabilize ingest, upload, WAL GC, and delete-time cleanup so stale acceleration state and retained WAL do not poison long-running servers
• improve cold and warm read performance with cached full-segment fetches, mmap-backed local reads, planned candidate scans, and faster JSON/raw response assembly
• expand memory observability with /_mem, runtime/process breakdowns, SQLite/runtime counters, high-water tracking, top-stream contributors, and operational docs
• harden GH Archive demo ingest and packaging/release workflows, including package smoke coverage and release documentation

Verification

• bun run verify
• bun run test:conformance
• bun run test:conformance:local
• bun run test:node-local-package
• bun run test:bun-local-package
• bun run test:bun-server-package
• npm pack --dry-run ./dist/npm/streams-local
• npm pack --dry-run ./dist/npm/streams-server

Notes

• remote soak and live-server work was validated during development
• PR remains draft and should not be merged yet