refactor: retire internal/logger; route scan observations through diagnostics

[fredbi]

The legacy debug logger (internal/logger.DebugLogf / UnsupportedTypeKind) and the dead slog logger on common.Builder predated the diagnostics system and were strictly less useful than it: unstructured, position-less, and written to stdout/stderr regardless of whether the caller wanted them.

This removes that machinery and folds its genuinely-useful output into the single Options.OnDiagnostic surface:

• pure control-flow trace and totally-normal drops (unexported field, swagger:ignore, anonymous field, untagged type) are deleted outright;
• "I dropped your config-filtered package/route" becomes a Hint (scan.ignored-by-rules / scan.ignored-by-tag);
• "I cannot model this Go type" becomes a Warning (validate.unsupported-go-type) — this closes a real gap, as those warnings previously escaped OnDiagnostic entirely;
• four stray log.Printf("WARNING...") lines (DeclForType/PkgForType defaults, swagger:model/response malformed names, swagger:enum with no values) are likewise routed through diagnostics.

Production code now makes zero writes to stdout/stderr, keeping codescan embeddable in a TUI or a WASI/WASM host.

Options.Debug is retained as a documented deprecated no-op for API compatibility. The diagnostic surface needed by OnDiagnostic callers is re-exported at the root (Diagnostic / Severity / Code aliases + Severity* constants) so consumers need not import internal packages.

Zero golden drift: diagnostics never alter the emitted spec.

Change type

Please select: 🆕 New feature or enhancement|🔧 Bug fix'|📃 Documentation update

Short description

Fixes

Full description

Checklist

• I have signed all my commits with my name and email (see DCO. This does not require a PGP-signed commit
• I have rebased and squashed my work, so only one commit remains
• I have added tests to cover my changes.
• I have properly enriched go doc comments in code.
• I have properly documented any breaking change.

[codecov]

Codecov Report

❌ Patch coverage is 79.66102% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.98%. Comparing base (795e662) to head (d406387).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
internal/builders/schema/schema.go	52.63%	8 Missing and 1 partial ⚠️
internal/builders/schema/allof.go	0.00%	2 Missing ⚠️
internal/builders/schema/embedded.go	75.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (79.66%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
+ Coverage   81.89%   81.98%   +0.09%     
==========================================
  Files          80       79       -1     
  Lines        7468     7440      -28     
==========================================
- Hits         6116     6100      -16     
+ Misses       1007      994      -13     
- Partials      345      346       +1     

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