Bound multiline flow span metadata

[jskoiz]

Summary

• Replace per-byte flow SourceMark metadata with compact affine source segments.
• Cap retained flow metadata at 4,096 segments while retaining normalized flow parsing and a safe final-tail fallback for oversized metadata regions.
• Add a bounded multiline-flow metadata regression and an exact physical-source-location regression.

Closes #77

Verification

Local proof:

• cargo fmt --all --check
• git diff --check
• cargo check --locked --lib
• cargo clippy --locked --lib -- -D warnings
• cargo test --locked --lib — 67 passed
• cargo test --locked --test diagnostics — 44 passed
• cargo test --locked --test yaml_test_suite multiline_flow — 2 passed
• cargo test --locked --test dos_hardening multiline_unterminated_flow — 2 passed

Hosted/merge state:

• This is a draft PR; hosted CI has not been observed in this task.
• The PR has not been merged.

Notes/Risks

• Exact source locations are retained through the bounded segment budget; after overflow, spans use the first omitted source boundary, except that the final flow tail remains exact. Parsing behavior and normalized flow text are unchanged.
• No exploit payloads were added.