refactor(tests): split source-map spec by functional domain

[luojiyin1987]

Summary

Split the 52 KB __tests__/source-map.spec.ts (1273 lines, 150 tests) into 9 spec files under __tests__/source-map/, one per functional domain. This is the follow-up promised in #101 ("Splitting source-map.spec.ts remains a separate follow-up change").

The production code already modularized into build-source-map.ts, code-segments.ts, recording-extension.ts, errors.ts, and types.ts. The tests now match that shape.

Layout

File	Domain
supported-fields.spec.ts	documented supported fields smoke
text.spec.ts	text.value mapping + non-contiguous ranges
inline-code.spec.ts	inlineCode.value
code.spec.ts	code.value
url.spec.ts	link / definition url fields
range-contract.spec.ts	range semantics: atomicity, zero-length points, line/column, argument validation
ownership.spec.ts	split siblings and owned-but-unmapped nodes
errors.spec.ts	error lifecycle + range resolution regression matrix
ast-parity.spec.ts	parseMd vs parseMdWithSourceMap parity corpus

Guarantees

• Zero runtime change and zero public API change.
• Describe titles are byte-identical; only file paths change in test reports.
• Test bodies moved verbatim. The only edits are import paths (./helpers → ../helpers) and two inline require('./helpers') calls in parity tests converted to normal imports.
• The existing 4 specs in __tests__/source-map/ are untouched.

Verification

Before: 14 suites / 1961 tests. After: 22 suites (−1 +9) / 1961 tests, zero failures. test:types passes.