[#3c] Extension reactor: forced-compat lane, hostcall reactor mesh, warm isolate pool

[quangdang46]

[#3c] Extension reactor: forced-compat lane, hostcall reactor mesh, warm isolate pool

Sub-issue of #3. Operational/perf surface needed once jcode is dispatching extension hostcalls at meaningful volume. Only ship after #3a/#3b unless there's a specific need.

Hard dependencies

• #3a and #3b must land first.

1. Hostcall compatibility-lane forced-compat switch

• Add a slow "compat lane" for hostcall dispatch alongside the default fast path.
• Global flag: --ext-compat-lane all (force every extension into compat lane until reverted).
• Per-extension setting: ext.compat=true in settings.json.
• Each forced state must record reason_code and triggered_by (mirror Extension trust lifecycle and kill-switch #15 ledger format).
• Use case: a hot-path bug in the reactor is found in production — operators can degrade to compat lane in one command without disabling extensions wholesale.
• Reference: pi_agent_rust → "Hostcall compatibility-lane emergency controls".

2. Deterministic hostcall reactor mesh

• Single-shard SPSC channel v1 (multi-shard v2 if profiling justifies).
• Bounded queue with backpressure: when full, the extension gets a denied hostcall response with code: "backpressure".
• Global drain on shutdown so no hostcall is lost mid-flight.
• Optional NUMA slab pool feature-gated behind numa — explicitly not required unless real workloads show NUMA pressure.
• Reference: pi_agent_rust → "Deterministic hostcall reactor mesh with optional NUMA slab pool".

3. Warm isolate pool + startup prewarm handoff

• Pre-instantiate N QuickJS isolates at startup (configurable; default 0 for batteries-off behavior).
• Hand the warmed isolate to the loader when an extension is first invoked → faster cold-load (target P95 < 50 ms).
• Pool sizing exposed in settings; observable via jcode doctor --only extensions.
• Reference: pi_agent_rust → "Warm isolate pool + startup prewarm handoff".

Out of scope

• Math-substrate (CUSUM, conformal, PAC-Bayes, etc.) — pi_agent_rust ships these but they are research surface with no direct user product. Do not import.

Acceptance criteria

• --ext-compat-lane all flips all extensions to compat path; jcode doctor reports the override.
• Reactor backpressure delivers a structured denied error rather than hanging.
• Cold-load P95 with prewarm pool of 2 measurably faster than without.

Cross-references

• Parent: TypeScript/QuickJS extension runtime for user-defined tools, commands, and hooks #3
• Siblings: #3a, #3b
• Related: #15b (lane-state ledger entries)

[quangdang46]

Sub-issue of #3

This is a numbered sub-issue of the Extension runtime epic (#3). The full implementation plan is at #3.

This sub-issue's scope (#39)

Reactor: forced-compat lane, hostcall reactor mesh, warm isolate pool.

Order

#27 → #28 → #29

Estimated scope: L each — depends on prior in chain.

[quangdang46]

Closed as part of #3 deferral — extension reactor mesh.