Remove dead public code in core crates

[0xh3rman]

Removes 87 public items that are unreachable from every core workspace binary, test, and bench. 48 files, +21/−461.

1. Found with cargo-hawk reachability analysis over the core workspace, with gemstone and primitives excluded as mobile-facing API boundaries; each finding was additionally checked against iOS/Android generated bindings, feature-gated tests, and macro codegen before removal
2. Per git history, 41 items are refactor leftovers (last callers removed by the repository-trait migrations and similar refactors) and 46 never had a caller in any commit (wrappers added alongside the form that is used, unused encode/decode codec halves, unwired testkit fixtures)
3. The gem_encoding protobuf decoder still consumes fixed32 wire fields for cursor correctness; only the never-read payload accessor is removed
4. Verified: cargo check --workspace, cargo clippy --workspace -- -D warnings, cargo test --lib --bins --workspace --features unit_tests (2319 passed), and a hawk re-run reporting zero remaining dead public items