Implement pre-emit/post-emit diagnostic count mismatch detection (TS-1)

[Copilot]

The test harness was missing the TS-1 internal diagnostic that detects when emit causes the checker to discover new semantic errors via the emit resolver. This was stubbed out with comments referencing the TypeScript implementation but never ported.

Changes

• internal/diagnostics/diagnostics.go — Add NewMessage() constructor for creating Message objects with arbitrary code/category/text, needed for the synthetic TS-1 diagnostics
• internal/testutil/harnessutil/harnessutil.go — Replace commented-out comparison logic with working implementation:
  • Collect diagnostic counts (program + syntactic + semantic + global) before emit
  • Emit, then re-collect the same diagnostics after emit
  • If counts differ, use the shorter list as the base result and append a TS-1 mismatch diagnostic with excess diagnostics as related info
  • Declaration diagnostics are excluded from the comparison to avoid false positives from GetDeclarationDiagnostics triggering the emit resolver as a side effect (Go shares checker state within a single program, unlike TS which uses two independent programs)
  • Skip comparison for large test suites (≥100 files) or when skipLibCheck && declaration are both set
• Baselines — The three target .diff files are removed (output now matches TypeScript reference), and their entries are removed from submoduleTriaged.txt

Example output

error TS-1: Pre-emit (1) and post-emit (5) diagnostic counts do not match! This can indicate that a semantic _error_ was added by the emit resolver - such an error may not be reflected on the command line or in the editor, but may be captured in a baseline here!
!!! related TS-1: The excess diagnostics are:
!!! related TS2304 file.ts:1:18: Cannot find name 'role'.

[jakebailey]

I think this is technically a dupe of #1301 but it's been a while so I think we don't need some of the fixes in that PR?