Extract shared boundary_common utilities from duplicated frontend code

[devin-ai-integration]

Summary

Seven boundary language frontends (Clojure, Crystal, D, Hare, Nim, Odin, VB) plus ECMAScript all duplicated identical scaffolding for file I/O, artifact assembly, boundary JSON extraction, and main-function injection. This PR factors that boilerplate into a new compiler::boundary_common module, eliminating ~250 lines of repetition across 8 files.

New shared helpers in boundary_common:

// Generic file-read → parse dispatch
pub fn parse_file_with<F>(path, parse_source: F) -> Result<UnifiedModule, String>

// File-read → parse + boundary → CompileArtifact
pub fn parse_artifact_with<P, B>(path, parse_source: P, extract_boundary: B) -> Result<CompileArtifact, String>

// Semantic + boundary assembly (no I/O)
pub fn artifact_from_source<P, B>(src, parse_source: P, extract_boundary: B) -> Result<CompileArtifact, String>

// First-line JSON extraction with configurable comment prefixes
pub fn extract_boundary_from_comment(src, prefixes: &[&str]) -> Option<BoundaryModule>

// Append default main() if absent
pub fn ensure_main(decls: &mut Vec<Decl>)

Each frontend now declares only a BOUNDARY_PREFIXES const and its language-specific parsing logic, delegating everything else to the shared module. Behavior is unchanged—all existing boundary tests pass.

Link to Devin session: https://app.devin.ai/sessions/044d1730fc5f41e2ae796230b73a3482
Requested by: @undivisible

[devin-ai-integration]

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