docs(examples): add a deterministic Node.js/npm stable-v1 integration recipe (#122)

[wufangyong973]

Closes #122

Mirrors the preserved Python/pytest recipe as examples/node-npm, but on the current stable ExecSurface v1.0.0 rather than the historical Alpha.5 pin. The recipe is a reproducible integration demonstration, not independent external validation or a safety certification.

What changed

• examples/node-npm/fixture/{test.js,fixture.json,package.json}: a one-file, zero-dependency Node fixture that asserts a sum invariant and only writes a harmless drift.txt when the public write_extra flag is true.
• examples/node-npm/run.js: a node:-built-ins-only runner that learns and checks the same real node test.js command, retaining an unchanged PASS (exit 0) and a deliberate-write REVIEW (exit 10) plus native command receipts, baseline immutability and verdict JSON. It targets the stable 1.0.0 binary (asserts execsurface --version) and the unchanged built-in review policy.
• examples/node-npm/README.md: copy/paste walkthrough, expected exit codes, the controlled drift, and the current Linux x86_64 native-ptrace support boundary.
• examples/README.md: registers the Node.js/npm recipe in the example-selection table as a current-v1 entry.

No changes to observer, normalization, baseline, lockfile, policy, verdict or GitHub Action semantics. The fixture is private to this repo and needs no network during the measured run.

Local verification

• node --check examples/node-npm/run.js passes.
• Fixture left + right = 5 invariant holds.
• node test.js with write_extra:false exits 0 and writes no drift.txt.
• node test.js with write_extra:true exits 0 and writes drift.txt with the documented controlled fixture drift content.

Full end-to-end learn/check assertions require the stable v1 Linux x86_64 execsurface binary with native ptrace; the runner encodes the expected verdict JSON shape (target.exit_code / target.signal / policy.source / verdict / findings[].change / effect_kind / evidence.effect.target.value) verbatim against the v1 schema in crates/execsurface-policy/src/lib.rs.