feat(core): add embedding hosts, persistent REPL state and bounded identities

[fffonion]

Summary

• Expose language-neutral embedding-owned CLI host catalog/registry and per-VM initialization services while retaining existing CliRuntime and SourcePlugin contracts.
• Admit checked native REPL modules/options, preserve persistent root locals while compacting callee slots, and synchronize exact root schemas independently of aggregate backend metadata.
• Preserve expression evaluation order and legacy result precision; add opt-in fixed-budget pruning of expired callable identity records without collecting resources or changing VM/native layout.

Base and scope

Depends on #33 (feat/generic-typed-frontends, accepted foundation d8c21caf8f91a8e5ceaffa10ea0b1473cdd4f43b). This PR adds seven reviewed commits and 28 changed paths. Its head is 77cdd0441b83baf536c711c5d9e797a9ee7eb093.

No TypeScript-specific Core implementation, new Value/schema variant, ordinary array semantics change, builtin-ID reassignment, VMBC/native/AOT version change, required public field or required plugin method is introduced. Resource/capture ownership and existing stale/foreign-callable rejection remain unchanged.

Verification

• Complete default workspace --lib --tests, parallel and serial: each 2304 passed, 13 ignored, zero failures; exact package/target/test/status sets match across 55 Cargo targets.
• Passed tests comprise 2287 business cases, 15 helper-reference cases and two outer trybuild harnesses. Separately, 14 compile-fail and two compile-pass fixtures match expectations.
• Current strict workspace/all-targets check and Clippy, fmt, focused no-JIT/no-runtime tests, and nine directly executed external SDK fixture tests pass.
• Real interpreter/JIT/AOT, persisted Named/optional root state, 257 sequential callee locals, live aliases/callbacks, metadata history bounds and native layout controls pass.
• Independent task reviews and whole-change final review: CLEAN.

Doctests, browser/wasm32 execution, additional feature combinations and the unsupported full disabled-backend no-JIT CLI suite are not asserted by the local matrix. Remote CI is pending. The nested external fixture harness is filtered locally; its nine tests run directly.

This Core producer PR does not claim completion of downstream TypeScript algorithms, generators or transparent array lifetime management.

[fffonion]

Consolidated into #33 at 7e5566b, organized into four scope commits. The final source tree is unchanged from reviewed 77cdd04; the original branch is retained for provenance and rollback.