fix: suppress stale-baseline findings and prepare v5.16.3

[thedoublejay]

Summary

• Suppress structural PR-review findings when baseline validation establishes that the workspace index or base ref is stale.
• Add regression coverage for a stale persistent index and a local main branch lagging origin/main.
• Preserve --no-baseline-check as an explicit opt-out.
• Bump the workspace and website version from 5.16.2 to 5.16.3.

Root Cause

Baseline validation emitted warnings for stale refs and index SHAs, but report extraction still compared the unreliable baseline graph with the freshly indexed head. That could fabricate removed symbols and value_mirror_incomplete findings for values that do not exist at either reviewed head.

Key Decisions

• Fail closed for structural surfaces when HEAD alignment, upstream divergence, working-tree cleanliness, or a known indexed SHA mismatch makes the baseline unreliable.
• Preserve metadata, safety information, changed files, and per-surface availability while returning no structural findings.
• Keep a missing indexed commit record as warning-only because it does not prove that the graph is stale.
• Allow callers to accept the current baseline explicitly with --no-baseline-check.

Files Changed

Area	Change
PR review command	Classify baseline reliability and suppress untrustworthy structural deltas
Regression tests	Cover stale persisted indexes and local base refs behind upstream
CLI reference	Document suppression and the explicit override
Release metadata	Bump Cargo and website packages to 5.16.3
Changelog	Document the stale-baseline correctness fix

Cross-Repo / Rollout Impact

None. This changes Gather Step report behavior only. Consumers may see empty structural sections plus an explicit warning when a baseline is known to be unreliable.

Verification

• cargo test -p gather-step --lib commands::pr_review::tests -j 1
• cargo check --workspace -j 1
• cargo clippy --workspace --all-targets -j 1 -- -D warnings
• cargo fmt --all -- --check
• git diff --check

Follow-ups

None.

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