Extract duplicated diff merge algorithm into shared module

[glehmann]

The diff merge algorithm (findBestInsertForDelete / buildInitialPairs / emitLines / mergeModifiedLines et al.) is copy-pasted across three files with slight, mostly-accidental divergences:

• src/browser/lib/diff-worker.ts
• src/browser/ui/diff/utils/parse.ts
• src/api/diff.ts

Plan:

1. Create src/shared/diff-merge.ts — extract the shared types (UNPAIRED, ReplaceKey, Line, ParseOptions, defaultOptions, Hunk, SkipBlock) and shared functions (calculateChangeRatio, changeToLine, buildChangeIndices, findBestInsertForDelete, buildInitialPairs, buildUnpairedDeletePrefix, hasUnpairedDeleteBetween, emitNormal, emitModified, emitLines, mergeModifiedLines, isSimilarEnough).

2. Unify to one correct findBestInsertForDelete — use array-index bounds (already fixed forward-only in Fix diff merge algorithm pairing deletes with wrong inserts across sections #247). The parse.ts variant using lineNumber bounds is a bug, not intentional.

3. Fix insertSkipBlocks in api/diff.ts — its fallback context ?? hunk.content shows a raw diff header to users (e.g. @@ -48,10 +48,16 @@). Should match the other two: "N lines hidden".

4. Update all three files to import from the shared module instead of defining their own copies.

5. Keep per-file: parseHunk (each has its own hunk-parsing wrapper), mergeAdjacentLines (only parse.ts uses it), insertSkipBlocks (different fallback logic that will be unified), worker infrastructure, and high-level parsing entry points.