AIS-5214: Clamp ending overlap in add/remove fold

[nguyenlc1993]

Summary

• Fixes the add/remove overlap fold in concatChanges (src/helpers.ts), which could drop a character from the change list.
• beginningOverlap and endingOverlap were computed independently. When the boundary character repeats (' ' vs ' '), the same character counted toward both, so getNonOverlap trimmed it twice.
• endingOverlap is now clamped to minLength - beginningOverlap where it is computed. The trailing common-suffix push already applied this clamp; the trim did not.
• Adds 3 regression cases in tests/index.test.ts. Each pins the exact getChanges() output and runs assertValidTracker.

Repro (before this change)

case	changes	rebuilt from changes	actual
' 0'.remove(2,3).add(2,'😊 ').remove(0,5)	[' ', [1,'0']]	original ' 0'	' 0'
'aa'.add(0,'aaa').remove(3,5)	['aa']	modified 'aa'	'aaa'
'aab'.add(0,'ab').remove(2,5)	['ab']	original 'ab'	'aab'

Impact

• Both sides of the change list: when the add is longer than the remove, the modified side breaks too, not only the original side.
• audio-server: its engine cache rehydrates trackers from getChangeChunks(). Chunk lengths are derived from the change list, so index mapping on those rehydrated trackers can be skewed.
• Existing cache entries: entries written by older versions keep their bad chunks until they are evicted.

Test plan

• The new cases fail before the fix, on the exact broken arrays shown above
• npm test: 180/180 pass
• npm run fuzz: 5000 iterations pass
• npm run build succeeds

Follow-up

• Release v1.4.3 after merge.
• Go port (ai-platform tts-service/pkg/stringtracker): apply the same clamp, add the 3 cases as fixtures, regenerate the golden oracle, and bump @speechifyinc/string-tracker in audio-server.

🤖 Generated with Claude Code

https://claude.ai/code/session_01DZPwQoG2HtFHPnTDvsncrR