[CTX-0031] chore(publish): renamed gc-arena fork for registry path (Option F)

[Xuepoo]

Priority: P2 | Area: area:packaging | Labels: chore,P2,area:packaging | Milestone: v0.2.0 | Task: CTX-0031 (phodopus side of bitty CTX-1262)

Refs bitty-terminal/bitty#1512 (Phase A follow-up, Option F). Publish-PREP only: no cargo publish run, no merge in this session. Owner publish order after merge: phodopus-gc-arena-derive, then phodopus-gc-arena, then phodopus and phodopus-util.

Scope: renamed registry fork of pinned gc-arena rev 5a7534b as phodopus-gc-arena plus phodopus-gc-arena-derive (version 0.5.3 under new names, library name gc_arena preserved, source verbatim except manifests); workspace dep repointed to versioned path dep; version-check split into two groups; unsafe-ledger and markdown gates scope the fork; gc-strategy, ledger scope, CHANGELOG, and release.toml updated; fork UI snapshots refreshed for current toolchain with trybuild pinned for MSRV 1.85. Zero phodopus source changes (all use gc_arena paths unchanged).

Verification:

• just check green (fmt-check, typecheck, clippy with scoped fork allows, workspace tests, links, unsafe-ledger, actionlint, markdownlint).
• just version-check green: 0.2.0-alpha.1 plus fork 0.5.3. just msrv green.
• cargo test --workspace --all-targets green (fork 24 integration plus 7 doc-tests included).
• cargo publish --dry-run -p phodopus-gc-arena-derive green.
• cargo publish --dry-run -p phodopus-gc-arena and -p phodopus fail only on registry-missing fork deps (expected pre-publish order, not manifest defects); git-dep blocker is gone. No cargo publish without --dry-run was run. Token untouched.