Rebase React Compiler loc.index on real source-file start to fix span inversion across shared SourceMaps

[Copilot]

This addresses incorrect span reconstruction when React-Compiler-mutated nodes fall back to loc: forward conversion treated offsets as if every file started at BytePos(1), and reverse conversion rebased using a file start derived from program.span().lo. In multi-file SourceMaps and files with leading trivia/shebang, this broke source-map fidelity and could push recovered spans out of file bounds.

• Forward conversion basis is now explicit and file-correct

  • Added source_file_start_pos: BytePos to ConvertCtx and ConvertResult, passed explicitly into convert_program.
  • ConvertCtx::position now computes file-relative indices via:

    let offset = offset
        .saturating_sub(self.source_file_start_pos.0)
        .min(self.source_text.len() as u32);

  • Clarified invariant in docs/comments:
    • BaseNode.start/end/node_id = absolute SWC BytePos
    • loc.line/column/index = Babel-style file-relative location

• Reverse fallback keeps exact inverse semantics

  • Kept span_from_base fallback logic (source_file_start_pos + BytePos(index)) unchanged.
  • Expanded comments to document the inversion contract between absolute start/end and file-relative loc.index.

• API and caller plumbing now use real SourceFile.start_pos

  • transform/lint now accept source_file_start_pos: BytePos.
  • Threaded through transform_source/lint_source and parser helpers.
  • Updated in-tree call sites (crates/swc, fixture harness, example binary) to pass actual file start, avoiding silent BytePos(1) assumptions.

• Regression coverage for failure modes

  • Added tests for:
    • leading-comment and shebang inputs (loc-only index == 0 recovers file start)
    • shared SourceMap second-file transform (recovered mappings stay within that file’s range)
    • non-BytePos(1) round-trip invariant (absolute == start_pos + loc.index)
  • Updated one fixture expectation to match corrected round-trip behavior for JSX type arguments.