Add refine option for two-pass sub-token diffs

[krkarma777]

Summary

• New DiffOptions.refine (default false): after the primary diff, each adjacent delete/insert pair is re-diffed one granularity finer — word pairs by char, line pairs by word — so quick → quicker reports the shared text as equal and just +er as the change
• Entry merging is preserved across splice seams (no two adjacent entries share an operation); char mode ignores the flag (already finest)
• Shared src/entries.ts now holds the DiffEntry/DiffOperation types and the merging pushEntry helper that was previously duplicated in chars.ts and scan.ts

Test Plan

• 6 new tests: word→char and line→word refinement, char no-op, solo delete/insert passthrough, merge/order invariants, 600 refine fuzz round-trips
• Full suite 34/34, typecheck clean