perf(mcp): lazy server startup from fingerprint-keyed schema cache (#56832 extract-salvage)

[kshitijk4poor]

Extract-salvage of #56832 by @Vansh5632 — the PR's still-novel core (lazy MCP startup), with the schema-cache module cherry-picked under their authorship and the wiring re-derived onto main's current connect machinery.

Context — what this changes for users

Every desktop/dashboard start spawns EVERY configured stdio MCP child (Playwright, etc.) and fetches schemas, even for servers the session never touches — main's idle recycle only reclaims them after opt-in idle windows, never avoids the initial spawn. With mcp_servers..lazy: true (default OFF), a server whose config fingerprint matches a valid on-disk schema-cache entry registers its tools WITHOUT spawning; the child starts on first actual tool use.

Scope honesty — the full 4-group map of the original PR

• Lazy MCP startup: extracted here (the PR's real value; main had nothing equivalent).
• Runtime trim group: superseded by feat(mem): config-driven allocator trim with telemetry (#66355 salvage) #76905 (context.memory_trim — richer telemetry + lifecycle coverage).
• Session-DB perf group: superseded on main (read-only opens, session_count_by_source, schema-v23 last_active SQL helpers).
• mimalloc preload group: genuinely NOT covered by anything on main (allocator substitution at spawn ≠ runtime trim) but requires a maintainer security decision (allow-dyld-environment-variables entitlement + build-host mimalloc staging) and a CJS→ESM port — deliberately left out; flagged for human review.

Design (from #56832) + re-derivation notes

• Registration-from-cache in register_mcp_servers; miss/stale falls back to eager connect with write-through refresh.
• First tool use routes through _ensure_lazy_server_connected, composing with main's connect cooldown (A single failing stdio MCP server churns the whole MCP bridge — healthy servers' tools become "Unknown tool" at runtime #50394) and _server_connecting dedup — no duplicated connect path.
• resource/prompt utility handlers (list_resources/get_prompt) also connect-on-first-use — closes the gap flagged in the original sweeper review.
• check_fn stays a pure dict-membership: tools list as available pre-connect without triggering a spawn.

Review follow-ups folded (simplify pass)

• Phantom-tool reconciliation: cached tools the live server no longer offers are deregistered after first-use connect (were permanent registry ghosts burning circuit-breaker strikes).
• atomic_json_write + cache/ location + 0o600 (registry discovery-cache precedent); no secrets persisted (command/args/env only hashed into the fingerprint, never stored).
• Cache-load path runs the same prompt-injection description scan as the eager path (the cache file is user-writable JSON).
• Write-through skips byte-identical rewrites (flapping stdio servers no longer hammer the disk).

Verification

• 444 mcp tests green (28 lazy/cache incl. 4 new guards); ruff clean.
• Mutation checks: cache-read disabled → registration test fails; connect bypassed → 3 first-use tests fail; phantom-dereg disabled → its test fails; write-skip disabled → its test fails; all restored green.

Attribution mapping for Vansh5632 landed in #77492.

Closes #56832.