Implement Unicode 17 source-mapped case conversion

[lukewilliamboswell]

Fixes #52

Summary

• adds the Unicode 17 UAX update to basic-cli 0.20 #29 default word-boundary substrate and public Word range/cursor APIs used by titlecasing
• adds deterministic UnicodeData, SpecialCasing, CaseFolding, Cased, Case_Ignorable, Soft_Dotted, and combining-class case data
• adds complete-string lower, upper, technical R3 title, and full/simple case folding with explicit default, Turkic, and Lithuanian policies
• returns owned transformed text plus exactly one source fact per input scalar, including byte/scalar ranges, shape, and contextual provenance
• adds explicit atomic limits and structured errors; no operation normalizes input or output

Policy and API decisions

• titlecase uses the same un-tailored Unicode 17 UAX update to basic-cli 0.20 #29 Word transition core
• full folding selects C/F, simple folding selects C/S, and explicit Turkic profiles apply T overrides
• source facts are source-oriented so deletion mappings retain complete input identity
• mapping shape and contextual selection are orthogonal public semantics
• Case is complete-Str only because ordered contextual output requires replayable source context

[lukewilliamboswell]

Independent review record:

• Maxwell (architecture_plan): reviewed design changes, Word data/runtime fixes, and final allocation evidence; approved after all material findings were resolved.
• Sagan (workflow_tests): reviewed Case data and runtime/conformance contracts; required and re-reviewed empty-map rendering, exact payload accounting, SpecialCasing ambiguity guards, public API accessors, contextual negatives, and exact limits.
• Boole (issue_analysis): authored separate data/conformance scopes and did not approve those authored changes.
• final_integration_review (fresh Tera reviewer): independently reviewed the complete origin/main...HEAD range, ICU differential, signatures, performance evidence, and post-rebase Bidi/Word/Case conflict resolutions; final verdict approved for push/draft PR.

Post-rebase evidence: clean worktree, origin/main ancestor, nine good signatures, deterministic generation, full repository suite, ICU4J 78.3 differential, and combined allocation baselines all pass.