Inspector: versioned sessions, transport, and Runtime domain

Parent: #139

Establish the usable inspector attachment and protocol foundation.

Acceptance:

• Make JSGlobalContext inspectability opt-in and observable with JSC-compatible defaults/toggles.
• Define a versioned, embedder-owned in-process JSON transport with an explicit authentication boundary.
• Implement concurrent session attach/detach and deterministic context-retaining teardown.
• Implement Schema.getDomains, Runtime enable/disable/evaluate, structured protocol errors, execution-context events, and evaluation exception details.
• Add focused Zig tests and a real C compile-link-runtime transcript host.
• Add system-JSC differential coverage for public inspectability behavior.
• Publish the implementation commit, audit/differential evidence, and protocol documentation.

Completed by 71bc743. The pinned public inventory is 117/117 with zero pending symbols and 12 zig-js extensions. The combined system-JSC differential matches 34/34 rows (a8ecd5e83f9f35ca). Protocol version, transport/authentication boundary, lifecycle, supported commands, and limitations are documented in docs/inspector.md.

Completed and published in 71bc743. The protocol foundation, public inspectability controls, focused Zig lifetime/concurrency tests, real C transcript host, 34-row system-JSC differential, and inspector documentation are all checked in. Follow-on execution control remains in #153.