refactor: extract leveva-casemap leaf crate (+ proptest fuzzing)
Continue the leaf-crate carve-out (after leveva-message / leveva-patricia) by
moving the RFC 1459 case-folding module out of the monolith into its own
workspace crate:
- leveva-casemap: byte/slice fold, compare, hash, and the owned-key `fold`. It
imports only `core` (Ordering + Hasher) and is the lowest leaf in the string
cluster — string/ident/matching/channel/registry/whowas depend on it, it on
nothing. std-only lib, proptest dev-dep.
leveva re-exports it as leveva::casemap (`pub use leveva_casemap as casemap;`),
so every crate::casemap::* / leveva::casemap::* path across ~40 source and test
files is unchanged — no consumer churn.
Fuzzing: stable toolchain only (no nightly / no cargo-fuzz), so proptest is the
runnable fuzz tier, matching leveva-message. Added 7 consistency/inverse
invariants fed arbitrary bytes and arbitrary (incl. non-ASCII) strings:
fold ⇔ eq_ignore_case both directions, cmp==Equal ⇔ eq, cmp antisymmetry,
hash-agrees-with-eq (prefix-free 0xff marker exercised), fold idempotent +
UTF-8-safe + byte-length-preserving, high bytes (≥0x80) identity, and
to_upper∘to_lower identity on the uppercase domain over the full byte range.
Gate: cargo build --workspace 0 warnings; cargo clippy --workspace --tests
clean; leveva-casemap 13 (6 unit + 7 fuzz) and the full leveva suite (60
binaries) green.
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