Harden semantic validation and preserve source diagnostics

[LunaStev]

Summary

This change replaces Wave's minimal string-based semantic verifier with a program-aware validation pass that rejects invalid programs before LLVM code generation. It also introduces structured semantic diagnostics with source provenance so errors point to the actual declaration, expression, or imported file that caused the failure.

The work is the prerequisite semantic-validation gate for the v0.2.1-pre-beta RISC-V effort. RISC-V backend failures cannot be diagnosed reliably while invalid source programs are still accepted by wavec check, lowered differently by wavec build, or reported as backend panics.

Semantic validation

• Collect functions, externs, globals, aliases, structs, enums, methods, and generic declarations before validating bodies.
• Reject duplicate top-level symbols, local bindings, parameters, fields, variants, methods, aliases, and same-scope declarations while preserving nested-scope shadowing.
• Validate unknown, void, pointer, array, named, alias, struct, enum, and generic types in declarations, signatures, fields, casts, and expressions.
• Enforce void and non-void return contracts, missing-return detection, and function call argument compatibility.
• Reject break and continue outside loops.
• Support default parameters and generic function, struct, field, and method type substitution during pre-monomorphization validation.
• Diagnose incorrect generic arity before monomorphization, including generic errors originating in imported modules.
• Require mutable lvalues for input and mutation operations.
• Validate assignment, compound assignment, increment, decrement, explicit casts, contextual array literals, and addressed array literals.
• Reject integer literals that would be silently truncated.
• Reject invalid implicit pointer/integer conversions and require explicit casts where the language contract requires them.
• Enforce deterministic mixed-width floating-point rules.
• Detect duplicate match cases by evaluated numeric value while continuing to allow enum aliases.
• Reject aggregate formatting until Wave defines an explicit aggregate formatter contract.

Structured diagnostics and source provenance

• Add SemanticDiagnostic with an error code, message, top-level node index, primary span hint, label, note, and help text.
• Preserve the existing string-returning parser API as a compatibility wrapper around detailed diagnostics.
• Preserve imported source text and map each expanded top-level AST node back to its source unit.
• Validate the expanded AST before generic monomorphization so user-authored failures retain their original source location.
• Remove the previous error-message parsing and substring-based diagnostic locator.
• Resolve structured declaration, keyword, and identifier hints within the relevant top-level source scope.
• Correctly locate repeated identifiers, repeated return statements, duplicate declarations, and other repeated syntax.
• Report imported semantic errors using the imported file path, line, column, and highlighted span instead of the entry file at 1:1.

LLVM and control-flow fixes

• Stop generating statements after the current LLVM basic block already has a terminator.
• Mark unreachable conditional merge blocks as unreachable when every branch terminates.
• Mark exits from statically infinite, non-breaking loops as unreachable.
• Lower returns with implicit coercion only after semantic validation has established type compatibility.
• Add LLVM floating-point widening and narrowing casts for valid explicit float conversions.

Regression coverage and corpus updates

• Add regression coverage for invalid returns, missing returns, loop control, function calls, casts, lvalues, duplicate declarations, unknown and void types, array contexts, integer truncation, match duplicates, formatting, mixed float widths, generics, and diagnostic source locations.
• Verify that wavec check and wavec build reject the same invalid source before a backend panic or E9001 failure can occur.
• Cover repeated local declarations and returns, duplicate top-level types, imported unknown types, and imported generic arity failures.
• Update TCP and overflow samples to express narrowing conversions explicitly.
• Avoid aggregate formatting in the formatting regression corpus until the formatter contract is implemented.
• Ignore the repository-local .tmp planning directory.

User and developer impact

Invalid Wave programs now fail earlier with actionable semantic diagnostics instead of reaching target-specific LLVM lowering. Diagnostics remain attached to the original imported source, and valid programs using defaults or generics continue to pass validation. This establishes a stable frontend baseline for distinguishing language errors from upcoming RISC-V ABI, code generation, linking, and runtime issues.

Validation

• cargo fmt --all -- --check
• cargo test --locked --all-targets — 15 tests passed
• cargo clippy --locked --all-targets -- -D warnings
• cargo build --locked --release
• Wave end-to-end suite — 96 passed, 12 environment/architecture skips, 0 failed
• All 13 examples passed wavec check
• All 79 standard-library modules passed wavec check
• git diff --check