Expand the corpus to cover every accepted grammar shape

[StreamDemon]

Summary

• Adds six corpus fixtures covering the accepted grammar shapes that had none (crates/AGENTS.md rule: "add corpus tests for every grammar shape accepted"):
  • extern_onchain.sp — extern "C" (plain + async string-target), extern onchain mod, top-level onchain enum, onchain mod with a storage block.
  • casts_literals.sp — as cast chains, based integer literals with separators/suffixes (0xFF_FFu32, 0b1010_1010u8, u128/u256), float exponents (2.5e-3, 1e10f64), string escapes (\x41, \u{1F600}), char literals, lifetimes in generic params and reference types.
  • async_await.sp — async fn, .await, .await?, ? chains outside pipes, .context(...).
  • modules_use.sp — inline/file modules, use with brace lists, pub use re-export, contextual crate/super path heads.
  • attributes.sp — bare markers, derive lists, named args (@supervisor(strategy: "one_for_one", ...)), actor-handler @mailbox(capacity: 2048).
  • expressions.sp — struct literals (value position, shorthand init, nesting, parenthesized block-head escape), boolean/comparison operators.
• The harness now discovers tests/corpus/*.sp instead of maintaining a hard-coded list, so a new fixture cannot be silently skipped; an is-empty guard catches a moved or emptied corpus directory. Discovery is sorted for deterministic ordering.

Sixth and final sub-PR of the enhancement wave (PR #71 roadmap: "Corpus expansion for all accepted shapes"). Runs last so the fixtures exercise the wave's final code (span tokens, operator enums, attribute args).

Related Issue

None (PR #71 roadmap item).

Spec Sections Affected

None — tests only. Fixtures deliberately stay inside the parser's accepted subset documented in crates/AGENTS.md (no match/while/for/closures/spawn/emit, no unit-expression (), block-statement ; rule respected).

Checklist

• Code follows the Sploosh design principles (one way to do it, explicit over implicit, etc.)
• Documentation updated in relevant docs/ pages — N/A, tests only
• Tests added or updated
• All build targets still compile (if applicable)
• Spec-only PR (skip Build Targets section if checked)

Build Targets Tested

• cargo fmt --all -- --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace all green locally (54 tests, 13 corpus fixtures).

Test Plan

• parses_corpus_files discovers and parses all 13 fixtures (7 existing + 6 new); every fixture parses clean on the wave's final parser.

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Summary by cubic

Add six new corpus fixtures to cover every accepted grammar shape and switch the test harness to auto-discover all .sp files. Improves parser coverage and prevents new fixtures from being skipped.

• New Features

  • Added corpus files: extern_onchain.sp, casts_literals.sp, async_await.sp, modules_use.sp, attributes.sp, expressions.sp.
  • Covers: extern/onchain modules and enums, casts and numeric/string/char literals, async/await and ?, modules/use and re-exports, attributes with named args, struct literals and boolean/comparison ops.

• Refactors

  • Test harness now discovers tests/corpus/*.sp (no hard-coded list) and sorts paths for deterministic runs.
  • Added an is-empty guard to fail if the corpus directory is missing or empty.

^{Written for commit bbed8be. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 7 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Auto-approved: Test-only PR: adds six corpus fixtures covering grammar shapes and switches harness to auto-discovery. No production code changes.

Re-trigger cubic</sub>