refactor: structured diagnostics; one validation path; no console output

[jstet]

Closes #63.

What changes

src/diagnostics.ts

Export	Purpose
Diagnostic	{ code, severity, message, name? }, with a closed DiagnosticCode union (~40 codes)
ConversionError	what every library function now throws: code, subject (the question), details (all findings when it summarises several), cause
WarningHandler, consoleWarning	the default handler, and the only place the library writes to the console

SubsetViolation is now Diagnostic, so validateSubset findings also carry codes. This is additive.

One validation path. XLSValidator.rowDiagnostic() (types, choice lists) and appearanceDiagnostics() are what validateSubset collects, and what the converter throws or warns with. The converter's duplicates are deleted: validateRowType, assertChoiceList, assertFileChoices, UNIMPLEMENTED_TYPES, and its own appearance check. The converter doesn't run the full validateSubset, because that would add the LimeSurvey name gate to skipValidation conversions, which sanitize instead.

Warnings → onWarning:

• LstsvConfig.onWarning
• XLSLoader LoadOptions.onWarning
• new FieldSanitizer(onWarning), new TypeMapper(onWarning)
• convertConstraint(expr, onWarning)

All default to consoleWarning, so current behaviour is kept. Two specific changes:

• A dropped constraint is now a constraint-dropped warning; before, it was a bare console.error.
• The "Removed other choice" console.log is gone: that's the normal collapse, not a finding.

Errors: every throw reachable from the package root is a ConversionError with a code. XPathSyntaxError subclasses it (xpath-syntax).

Compatibility

• Additive API. The converter's error message text changed (it now uses the validator's wording), so tests assert code now. Consumers that match messages should switch to codes.
• TSV output unchanged (re-blessed, byte-identical).

Tests

tests/ts/unit/diagnostics.test.ts:

• the converter reports through onWarning, and not the console
• the sanitizer handler works
• seven throw sites produce the expected codes
• subject names the question
• a guard: no console.* outside diagnostics.ts and the CLI

Results: vitest 898 passed. npm run validate is clean. pytest: 37 passed.

README gets an "Errors and warnings" section.

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