feat: emit labeled MLIR at each pipeline stage

[hedgar2017]

• finalize_module returns Vec<(String, String)> with labeled MLIR snapshots at each dialect / pipeline stage
• --emit-mlir prints all stages (MLIR Dialect sol:, MLIR Dialect llvm:)
• --output-dir writes per-dialect files ({contract}.sol.mlir, {contract}.llvm.mlir)
• Standard JSON carries the full labeled list
• Missing LLVM dialect entry is an explicit error (not silent empty bytecode)

[Copilot]

Pull request overview

This PR enhances MLIR observability by capturing and emitting labeled MLIR snapshots for each compilation pipeline stage (e.g., Sol dialect pre-lowering and LLVM dialect post-lowering) across CLI output, --output-dir artifacts, and standard JSON.

Changes:

• Update solx-mlir::Context::finalize_module to return Vec<(dialect, mlir_text)> with per-stage snapshots.
• Emit/serialize stage-labeled MLIR in CLI (--emit-mlir), output directory writing, and standard JSON contract output.
• Update downstream consumers (Slang frontend + solx-core project/build structs) to store and use the staged MLIR representation, including an explicit error when the LLVM stage is missing.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
solx/tests/cli/output_dir.rs	Tightens --output-dir MLIR test to require multiple stage outputs and expected per-dialect filenames.
solx/tests/cli/emit_mlir.rs	Updates CLI test expectations to validate labeled stage headers and representative ops per dialect.
solx-standard-json/src/output/contract/mod.rs	Changes mlir output field from a single string to an optional labeled stage list.
solx-slang/src/slang/mod.rs	Switches Slang frontend to store staged MLIR output into standard JSON.
solx-mlir/src/context/mod.rs	Introduces dialect constants and returns labeled MLIR snapshots from finalize_module.
solx-core/src/project/mod.rs	Extracts LLVM-stage MLIR for compilation while retaining full stage list for outputs; adds explicit missing-LLVM-stage error.
solx-core/src/project/contract/mod.rs	Updates project-level contract representation to store staged MLIR.
solx-core/src/build/contract/mod.rs	Prints and writes per-dialect MLIR outputs for CLI and --output-dir; writes staged MLIR into standard JSON.
solx-core/src/arguments.rs	Updates CLI help text to reflect emitting MLIR at each pipeline stage.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated no new comments.