smoke(8al-node): load the Pi extension with the modules the Pi host provides

[DeusData]

Why

pr-smoke 8al-node imports the generated Pi extension in bare Node with an empty node_modules. That was a faithful stand-in for the Pi host only while the extension imported nothing but node: builtins. #1809 adds the first package import — typebox, Pi's schema library for tool parameters, a real Pi dependency (verified against pi-coding-agent 0.84.4 by its author) — and the probe can never resolve it:

Error [ERR_MODULE_NOT_FOUND]: Cannot find package 'typebox' imported from .../pi-node-probe/cbmem.mjs
FAIL 8al-node: generated Pi extension did not contain early-exit stdin errors

So the probe rejects a correct extension, and the failure message blames the wrong thing.

What

Give the probe what the host gives the extension:

• a Proxy-backed typebox stub in the probe's node_modules — any Type.X(...) returns a descriptor, so the probe never has to track which helpers the emitter uses; it only exercises the tool lifecycle (EPIPE handling, JSON authority), exactly as before
• an explicit allowlist: every import … from '<specifier>' in the generated file must be a node: builtin or a package on PI_HOST_PACKAGES (currently typebox); anything else fails the probe naming the specifier. The empty node_modules used to give that "no surprise dependency" guarantee by accident; now it is an asserted contract with a readable failure.

Main's extension (only node:child_process) is unaffected.

Verified

Ran only the 8al-node block against extensions generated by two binaries:

extension	probe	result
main@5802ccdd	old	OK
main@5802ccdd	new	OK
#1809 (b026a69)	old	ERR_MODULE_NOT_FOUND typebox — the CI red
#1809 (b026a69)	new	OK
#1809 + injected import lodash from 'lodash'	new	FAIL 8al-node: … packages the Pi host does not provide: lodash

Scope flags (CI change)

• gating: unchanged (same smoke step, same venues; the C-only Linux image still SKIPs as whitelisted)
• cost: negligible (two small files written into the existing temp probe dir)
• flake surface: none added — no network, no install, no version resolution
• trigger scope: unchanged

Unblocks the maintainer-side half of #1809 (its remaining red, test_agent_clients.c:1181, is the PR's own and has been handed to the author).