Share UTF-8 text snapshots and reduce text editing allocations

[elcritch]

Summary

• Store NimKit text in shared immutable UTF-8 snapshots with sparse rune and line checkpoints. Rich text uses compact style IDs and byte/rune ranges, and layout receives borrowed source spans through FigDraw.
• Create storage and TextView undo snapshots only when undo can use them. Transfer prepared snapshots into undo records, reuse the first snapshot for grouped edits, and keep marked-text delegate ordering and value-change notifications.
• Apply syntax highlighting with one ordered run-table update and one style-table compaction. Retokenize safe single-line SynEdit edits locally; use a full scan when lexical state may cross the edited line.
• Replace dense byte-to-rune maps and SynEdit's per-byte token cells with token ranges and monotonic UTF-8 endpoint cursors. Markdown code and blockquotes likewise map only token or inserted-prefix boundaries.
• Keep rune-based editing ranges and expose TextByteRange for parser/layout endpoints, with explicit snapshot conversions. Snapshots provide rune indexing/iteration without materializing a sequence; toRunes() remains available when a decoded sequence is useful.
• Add regression coverage for ordinary and gap storage, undo modes, UTF-8 and malformed bytes, CRLF, and nested blockquotes. Print diagnostic RSS for large styled text and undo modes.
• Record before/after standalone memory profiles and the public API review in TODO-utf8runes.md.
• Bump Merenda to 0.22.0 and update CHANGES.md.

Dependency

Depends on elcritch/figdraw#93: elcritch/figdraw#93

merenda.nimble points to FigDraw's feat/borrowed-utf8-text-spans branch with the existing siwin, sharedlib, and harfbuzz features. Atlas resolves that branch at ca08508. The FigDraw PR contains the borrowed-span API, compatibility fixes, an RSS benchmark, and FigDraw 0.42.0.

Verification

• atlas-run tests: 4/4 runners passed after the public API additions. Focused atlas-run tests tnimkit also passed (1/1).
• Standalone release/ARC profile versus parent dd933623 (median of three fresh processes): 8 MiB gap edit with undo disabled, +20,816 to +4,384 KiB RSS; enabled, +20,864 to +12,640 KiB; SynEdit tokenization of 8 MiB spaces, +81,968 to +8,208 KiB. Applying 2,048 styles over 1 MiB used +1,440 versus +1,312 KiB RSS, while operation time fell from about 0.336 to 0.006 seconds.
• With the public snapshot API, iterating 4,194,304 runes added 0 KiB RSS after setup; explicit toRunes() added 16,432 KiB. These are process RSS observations, not allocation counts.
• atlas-run tests --compile-only examples/all_compile.nim: 1/1 compiled.
• nim r tests/nimkit/styledruns.nim: passed. An earlier standalone 8 MiB / 8,192-run sample reported borrowed spans +400 KiB RSS and owned runs +12,240 KiB after storage. RSS is diagnostic; the shared NimKit runner can report zero growth when it reuses resident pages.
• FigDraw's native library build and four dynlib test files passed with Nim 2.3.1 in the dependency PR. No dynlib build was run for this Merenda update.