Port competitor test corpora (wave 2), extend parity targets, and ratchet a coverage gate

[konard]

Context

Issue #47: "We should have 100% tests coverage, which should copy most of test
cases from our competitors in each sector/scope" and "We should check that
nothing is deferred or left unimplemented in our vision and roadmap." Today 48
provenanced PARITY_FIXTURES exist (a small fraction of upstream suites), the
cargo llvm-cov job enforces no threshold, and docs/parity-roadmap.md still
defers SQL dialect and Delphi-specific coverage. See
requirements.md R-19/R-20 and
solution-plans.md S-15.

Blocked by: #02, #08, #12, #14 - this is the closing gate over the
features those issues add.

Scope

• Port the five highest-value suites identified in
  competitors-code-tooling.md:
  Coccinelle tests/ (.c/.cocci/.res transform triples), tree-sitter
  test/corpus + core test/fixtures/error_corpus, Semgrep
  tests/patterns/<lang>/, srcML test/parser/testsuite lossless round-trip
  cases, LibCST adversarial whitespace fixtures (runner-up: babel-parser
  fixtures). Sample per construct where suites are huge; document sampling so
  nothing reads as "covered everything" when it is not.
• Extend PARITY_TARGETS with the surveyed projects not yet tracked:
  ast-grep, Semgrep, Comby, GritQL, srcML, difftastic, Babel, SWC,
  OpenRewrite, Spoon, JavaParser, Rascal, Stratego/Spoofax, TXL, MPS,
  Coccinelle; GF, Universal Dependencies, LanguageTool from the
  natural-language survey; doublets-rs and links-notation storage gates.
• Ratchet coverage: record the current cargo llvm-cov line coverage, fail
  CI below the floor, raise the floor with each wave toward 100%.
• Roadmap audit (R-19): every deferral in docs/parity-roadmap.md is either
  implemented or tracked by an open issue with a link.

Acceptance criteria

• Each new parity target has provenanced, license-recorded fixtures and
  capability gates, like existing targets.
• Ported corpora include both round-trip and transform-expectation cases.
• CI fails when coverage drops below the recorded floor.
• Roadmap contains no untracked deferral.
• Changelog fragment added (bump: minor).

References

• Requirement: requirements.md R-19, R-20
• Solution: solution-plans.md S-15
• Part of We need to carefully compare all competitors in all scopes with out project, and make sure we have the richest features set #47; work lands on branch issue-47-76af108c0f24 (PR Finish issue #47 parity feature set #48).

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Filed from docs/case-studies/issue-47/proposed-issues/15-competitor-corpora-and-coverage-gate.md. Part of the implementation plan for #47.