PR-017 - PBS Const Evaluation and Dependency Rules

Briefing

declare const validation only checks explicit type and initializer presence. This PR implements constant-expression legality, dependency ordering, cycle detection, and initializer type compatibility.

Motivation

Const declarations are part of static semantics and must be deterministic across files.

Target

Const semantic validation layer.
Diagnostic coverage for constant expression failures.

Scope

Enforce allowed constant-expression subset.
Resolve const dependencies module-wide, independent of source-file order.
Reject cycles and unresolved const references.
Validate initializer type compatibility with declared const type.

Method

Build const dependency graph from top-level declarations.
Evaluate in topological order.
Emit stable diagnostics for disallowed forms and graph failures.

Acceptance Criteria

Non-constant initializer forms are rejected deterministically.
Cross-file const references resolve independent of source order.
Cycles are rejected deterministically.
Incompatible const initializer types are rejected.

Tests

Add const-expression positive/negative fixtures.
Add cross-file dependency and cycle tests.
Add type-compatibility tests.

Non-Goals

General compile-time function execution.
Runtime const materialization strategy changes.

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