feat: useful release-pinned docs on first use

[shtse8]

Outcome

• Default docs/api queries anonymously fetch public upstream docs at the immutable commit resolved from the requested release tag, using the existing fetch/cache owner.
• Registry packages and major-version docs sites remain explicit opt-in. No ambient credentials, redirects, or latest-version substitution on the default path. Git checkouts retain their resolved source identity.
• Add CLI/env/MCP offline controls, bounded upstream work, cache-write isolation, and text/JSON provenance including fetch failures.
• Measure real first-use defaults in an initially empty isolated supported cache before prefetch; enforce full-suite floors of 96/105 fetched and 60/105 package-only hybrid. Historical published scores remain labeled historical.

Validation

• Passed network-free benchmark harness tests (2), Python syntax checks, named-file rustfmt, git diff --check, version/capability/tagline checks.
• Rust compilation, workspace tests, clippy, docs build, and real benchmark remain CI-only per task instructions. No new score claimed before measurement.

Review/landing

• Architect final review is lead-owned after exact-head checks and benchmark readback. Do not release without the lead.