Report actionable authoring diagnostics without exposing internal bookkeeping

[noeltock]

Problem and change

Rejected authoring proposals and full plans now return stable diagnostic codes, source locations when verified, affected declarations/assets, and supported next actions through the existing report format. Coverage differences are bounded to 12 entries with totals and categories; exact coverage validation remains enforced.

Stale references retain their source path and node/reference identity without presenting foreign offsets as current locations. Unsupported native mappings explain their limitation, and filesystem dependency failures are distinguished from design decisions.

Validation

• 655 tests passed; 4 skipped, including the 34 focused proposal, diagnostics, and native adapter tests.
• Node support, typecheck, corpus hashes, build, private-reference check, and package dry run passed on Node 22 on macOS.
• The private-reference check used the existing npm lifecycle workaround (npm_config_script_shell=/usr/bin/true for that direct check only); all other checks ran normally.
• Independent review found a stale-location issue; it was corrected and covered by regression tests.
• Exact-commit CI passed: Node 20/22/24, package boundary, and WordPress 7.1 proof. This change makes no model-usability or manual WordPress acceptance claim.

Closes #64.