refactor(source-map): derive segment offsets from text nodes

[luojiyin1987]

What changed

• Derive valueStart from the target text node.
• Initialize segment storage by target node identity.
• Remove RecordingState.current and RecordingState.len.

Why

The global cursor duplicated data already present on each text node. It also coupled segment offsets to handler order.

Impact

Each text node now owns the data used to calculate its segment ranges. Parser output and source-map behavior stay unchanged.

Checks

• 1,849 source-map tests passed.
• pnpm exec tsc --noEmit
• pnpm run lint

Closes #84
Part of #86

_{Stack created with GitHub Stacks CLI}