fix: preserve streamed line starts inside a chunk

[lttlin]

Problem

layoutNextLine() and layoutNextLineRange() renormalize every continuation cursor as if it were starting from a fresh paragraph or chunk boundary. In mixed-script edge cases with a ZWSP break opportunity followed by collapsible whitespace, that can skip a real continuation segment and merge two streamed lines that layoutWithLines() keeps separate.

Root Cause

The line-start normalization helpers unconditionally skipped space, zero-width-break, and soft-hyphen segments even when the caller was resuming from an exact end cursor returned by the previous streamed line. That normalization is correct at the true start of a paragraph or chunk, but it is too aggressive for continuation cursors inside an existing chunk.

Fix

Only trim collapsible leading break-opportunity segments when the cursor is at the true paragraph start or chunk start. If the caller resumes from a later in-chunk cursor, preserve it exactly and continue streaming from that segment.

Validation

• cd /tmp/pretext_issue_121 && ~/.bun/bin/bun test src/layout.test.ts --test-name-pattern 'streaming canary preserves exact continuation cursors after a ZWSP break opportunity|layoutWithLines strips leading collapsible space after a ZWSP break the same way as layoutNextLine|layoutNextLine reproduces layoutWithLines exactly'
• cd /tmp/pretext_issue_121 && ~/.bun/bin/bun test src/layout.test.ts

Closes #121