fix: sync streaming line-break stepper with batch walker

[voidborne-d]

Fixes #121

Two mismatches between layoutNextLine/layoutNextLineRange (streaming) and layoutWithLines/walkLineRanges (batch) caused the streaming APIs to produce different line breaks for certain inputs involving mixed scripts, soft hyphens, and non-simple whitespace modes.

1. maybeFinishAtSoftHyphen premature fallback (pre-wrap + soft-hyphen)

When an overflow occurred at a breakAfter segment (preserved-space, tab, etc.) and a soft-hyphen pending break was active, the step function's maybeFinishAtSoftHyphen had a fallback path that used the soft-hyphen break even when the segment's breakableFitAdvances was null. The batch walker's equivalent (continueSoftHyphenBreakableSegment) returns false in this case, allowing the closer breakAfter opportunity to be chosen instead.

Fix: Remove the fallback from maybeFinishAtSoftHyphen so the general pending-break check handles it, matching the batch walker's check order.

2. normalizeLineStartInChunk over-strips spaces (chunked path)

normalizeLineStartInChunk skipped space segments at line starts, but the chunked batch walker (walkPreparedLinesRaw non-simple branch) does not strip leading spaces when starting a new line within a chunk. This caused the streaming stepper to skip a leading space that the batch walker kept, producing fewer lines.

Fix: Condition the space-skip on simpleLineWalkFastPath so only the simple path strips spaces (matching normalizeSimpleLineStartSegmentIndex and the simple batch walker).

Reproduction

Using the reproducer from #121:

const text = "بام  \u200DB     bا \u00ADb\u060C b\f \u061F\uD83D\uDE80\u061F\u0639 \u0631 \u672C \u061F\na a A\u200B \u8A9E \u8A9E\u200D\u062D"
const prepared = prepareWithSegments(text, font)
const batchLines = layoutWithLines(prepared, 56.57, 20) // 8 lines
const streamLines = collectStreamedLines(prepared, 56.57) // was 7 lines, now 8

Tests

Added 3 regression tests:

• pre-wrap soft-hyphen does not preempt a closer breakAfter opportunity
• layoutNextLine keeps leading space inside a non-simple chunk when the batch walker does
• mixed-script Arabic+CJK text keeps streaming aligned with batch layout

All 87 tests pass (bun test src/layout.test.ts).