Implement structured diagnostic policy and compatibility

Parent: #8
Blocked by: #25 and #26

Outcome

Create the shared diagnostic policy used by every compiler phase and expand supported diagnostics through focused Compatibility Oracle cases.

Acceptance criteria

• Phase modules report structured diagnostic facts rather than preformatted console strings.
• The policy preserves occurrence, FS code, effective severity, message, source range, ordering, output stream, suppression, warning level, promotion, exit behavior, and artifact-publication behavior.
• Parser and checker recovery can report multiple deterministic diagnostics without uncontrolled cascades.
• Culture, flat/full-path formatting, UTF-8 output, warning-as-error, per-code suppression, and per-code promotion are represented explicitly.
• Each supported language feature contributes positive and negative diagnostic cases to the generic conformance manifest.
• Prototype-only diagnostics are reserved for explicit unsupported envelopes or internal failures and cannot stand in for known FS behavior.
• Complete family-wide FS Diagnostic Compatibility remains visible as a final Compatibility Gate, not inferred from a few sample codes.

Decision inputs

Issues #2, #5, #15, and #16; ADRs 0004, 0005, 0008, 0014, and 0016.

This issue inherits the corpus-neutral acceptance policy in #8.

Start with the corpus-neutral acceptance policy in #8, then read the blocking issues #25 and #26 and ADRs 0004, 0005, 0008, 0014, and 0016. Trace how the generic conformance manifest and Compatibility Oracle cases are organized. Done means all listed diagnostic policy behaviors and family-wide FS compatibility gates are represented without prototype substitutions.