Extract a reusable generator diagnostics helper #185

Problem

Every generator in this repo hand-rolls the same diagnostic plumbing. QuantitiesGenerator declares four DiagnosticDescriptor fields (SEM001–SEM004) and LogarithmicScalesGenerator a fifth (SEM005), each with its own private Report helper:

private static readonly DiagnosticDescriptor InvalidScaleDefinition = new(
    id: "SEM005",
    title: "logarithmic.json scale definition is invalid",
    messageFormat: "logarithmic.json validation issue: {0}",
    category: Emit.DiagnosticCategory,
    defaultSeverity: DiagnosticSeverity.Warning,
    isEnabledByDefault: true);

private static void Report(SourceProductionContext context, string message) =>
    context.ReportDiagnostic(Diagnostic.Create(InvalidScaleDefinition, Location.None, message));

Nothing about that shape is Semantics-specific. Three problems come with it:

1. The boilerplate is copied per generator, and the IDs are allocated by hand — the next generator author has to grep for the highest SEM0xx in use.
2. GeneratorBase does not follow the scheme. It reports JSON parse failures as CONV001 in category "SourceGenerator" — a leftover from when it served only ConversionsGenerator, and inconsistent with everything derived from it.
3. AnalyzerReleases.Shipped.md / AnalyzerReleases.Unshipped.md are maintained by hand. With EnforceExtendedAnalyzerRules=true, RS2008 fires when a descriptor is not tracked there, so this is a real recurring cost and an easy thing to forget.

Proposal

Extract into the shared toolkit:

• A DiagnosticFactory / descriptor-registry type parameterized by prefix (SEM, CONV, whatever the consumer uses) and category, so a generator declares descriptors declaratively and reporting is one call.
• Convenience overloads for the recurring shapes: "metadata validation issue", "unknown reference in metadata", "referenced file not found" — the three that account for all five descriptors here.
• Optional Location support. Everything here reports Location.None, so a typo in dimensions.json gives no file or line. Since the metadata arrives as an AdditionalText the generator can map an offset to a Location, which would make these diagnostics genuinely navigable. Worth doing as part of the extraction rather than after.
• A check (test or build step) that every declared descriptor is present in AnalyzerReleases.*.md, so RS2008 is caught before it fires.

Acceptance criteria

• All SEM00x descriptors are declared through the shared helper; no generator declares a raw DiagnosticDescriptor.
• GeneratorBase's JSON-failure diagnostic uses the consumer-configured prefix and category — CONV001 / "SourceGenerator" is gone.
• Existing diagnostic IDs and message formats are unchanged (they are documented in CLAUDE.md and docs/physics-generator.md).
• Analyzer-release tracking is verified automatically.

Context

Part of #181. Closely related to #182 — the base generator's diagnostics are the worst offender.