Fix qualified range endpoints and literal recovery

[itsfuad]

Summary

Fixes #157.

• Parse bare qualified range endpoints without consuming the loop-body brace; retain imported bindings, source locations, and range-entry evidence.
• Use current grammar (Type.{...}, .{...}, and Enum::Variant with payload) and remove migration-only literal parsing and stored control-header state.
• Recover missing literal introducers with one local, nesting-aware scanner; preserve enclosing delimiters, following statements, and independent errors.
• Keep parser-owned BadExpr diagnostics intact in the resolver and publish invalid type evidence in the typechecker. Type-argument diagnostic wording requires parsed AppliedType.TypeArgs evidence.
• Add parser/semantic/pipeline regressions and executable positive/negative fixtures, update frontend documentation, and install portable investigation/planning skills.
• Includes author-added backlog documentation in separate commit afc5568; implementation is commit a145d64.

Delivered

• Author merged this PR on 2026-10-07 at 29eac4d7de54cb82babf880ea26cf876ba6b1d92; Disambiguate qualified range endpoints from loop bodies #157 is closed with its acceptance checklist complete.
• All applicable PR checks and post-merge main CI passed, including six native Linux/macOS/Windows amd64/arm64 jobs and bundled source fixtures. Post-merge CI: https://github.com/PeeperLanguage/compiler/actions/runs/37576611310.
• Disambiguate qualified range endpoints from loop bodies #157 and PR Fix qualified range endpoints and literal recovery #159 are both Done in Peeper Roadmap. Project access was granted during post-merge reconciliation; the earlier access blocker is resolved.
• Struct literals follow current grammar. Add LSP struct literal snippet completion after Type. #134 remains a standalone deferred LSP completion feature; there is no struct-literal migration or publication prerequisite.

Production ownership

• Production path: parseForStmt -> parseIndexOperand / parseRangeExpr -> Pratt expression parser -> existing binding, typechecking, THIR, MIR, and LLVM owners.
• recoverBracedLiteral is called by prefix recovery, identifier/qualified-path suffix recovery, and parsed generic-path recovery. It centralizes non-obvious delimiter/body ownership rather than adding a second parser.
• literalFieldFollows is reused by header disambiguation and argument-comma recovery. Both helpers have current non-test production callers.
• Reuses existing precedence handlers, parseBracedItemList, registered BadExpr, diagnostic codes, and semantic evidence publication.
• Removes Parser.isControlHeader, parseExprWithControlHeader, variantLiteralPrecedesControlBody, migration-only literal branches, and the obsolete struct-field pass-through layer. No unused production API or experimental scaffolding remains.

Design and behavior

• Previously, values::Zero..values::One { ... } could treat the body as a malformed enum payload and cascade into missing delimiter/return diagnostics. Qualified control headers followed by standalone blocks had the same ambiguity.
• Valid literal braces now have explicit introducers. Recovery carries the caller's delimiter locally; operators inherit it and calls/grouping/indexes/literal values establish their own boundaries.
• Header recovery requires invalid field-shaped contents and a following body or expression continuation. Adjacent brace groups alone do not classify a literal.
• Qualified paths receive honest struct/enum correction alternatives; parsing never infers enum identity solely from ::.
• Preserves node registration/spans, later valid syntax and bindings, independent diagnostics, generic-token splitting, and downstream range/lowering invariants. Unknown-node invariant panics remain intact.
• Unsupported syntax remains rejected. Diagnostic wording and recovery shape intentionally replace migration-specific handling and redundant semantic cascades.

Validation

Passed on final compiler source, before the documentation-only backlog commit:

go test -count=1 ./internal/frontend/parser ./internal/semantics/resolver ./internal/semantics/typechecker
CCACHE_DISABLE=1 go test -count=1 ./...
go run ./scripts/bundle.go
PEEPER_BIN="$PWD/build/bin/peeper" CCACHE_DISABLE=1 go test -count=1 ./x_test -run '^TestFixtureContracts/(negative_literal_recovery|negative_variant_literal_control_header|runtime_struct_literal_dot|for_qualified_range_endpoints)$'
git diff --check
git diff --cached --check

The focused parser/resolver/typechecker check passed again during read-only review (0.216s / 0.033s / 1.678s). Final targeted bundled fixtures passed in 0.469s.

Earlier Step 7 validation passed before the final diagnostic-only type-argument guard; these results are inherited, not rerun for that guard:

go test -count=1 ./internal/semantics/analysis ./internal/ir/thir ./internal/ir/mir
go test -count=1 ./internal/pipeline -run '^TestPipelineRecordsQualifiedRangeEntryAcrossTargets$'
CCACHE_DISABLE=1 go test -race -count=1 ./...
go vet ./...
PEEPER_BIN="$PWD/build/bin/peeper" CCACHE_DISABLE=1 go test -count=1 ./x_test

Full bundled fixtures passed in isolation (18.405s). Pipeline regression covers 386/amd64 binding, imported ownership, types, guaranteed entry, missing-return behavior, and lowering/object contracts. Disposable peeper check probes exercised targeted literal diagnostics and valid adjacent-block controls.

At PR creation, preflight verified remote main was 03a3272, reviewed both included commits, and confirmed implementation-source diff checks pass. Whole-branch git diff main...HEAD --check reports the two-space Markdown line breaks in the author's separate TODO.md commit; the same check excluding TODO.md passes. No compiler source changed after validation. Cross-platform native execution subsequently passed in PR and post-merge CI.

Diff sanity

• Peeper code review: Standards 0 findings; Spec 0 findings.
• No unrelated production changes, duplicate semantic decisions, dead code, or temporary/debug instrumentation introduced.
• Shared recovery helpers satisfy reuse and invariant-protection rules. Documentation records current grammar and actual recovery ownership.
• Author confirmed manual review-marker removal; the agent did not remove it.

Risks and follow-up

• Body ownership and enclosing delimiters must remain protected; do not restore stored header mode or indiscriminate two-brace lookahead.
• Run bundle-dependent fixtures separately from other expensive suites: an earlier concurrent run hit missing/invalid runtime-archive evidence; isolated reruns passed.
• Related existing follow-up: Add LSP struct literal snippet completion after Type. #134 (LSP struct-literal snippets for current syntax). No new tactical bridge introduced.
• Milestone: 0.2 Language Foundations. Roadmap project sync is complete.
• Merge requires passing applicable CI and approval from a non-author human collaborator.