embedder: give host resource registrations independent lifetimes

[lannbot]

Summary

Fixes #330.

• Allocate independent host-resource registrations for each own lowering of a host implementation object, even when backing JS object identity is equal. Returning/dropping one registration leaves the other usable; each owning drop invokes disposal independently.
• Allocate fresh call-scoped host-originated borrow registrations. Preserve existing guest-resource wrapper transfer and canonical lender protection.
• Remove the obsolete identity-deduplication/deferred-disposal policy, its cleanup-error machinery, and its dedicated fixture/tests. Add real-Wasm independence, return, overlap, and failure-cleanup regressions.
• Clarify the embedder contract and advance protocol from 0.3.2 to 0.4.0 (breaking/protocol); the lockstep runtime packages stay at 0.6.8.

Semantics

Resource identity is distinct from backing-data identity. The pinned Component Model creates fresh resource handles and independently calls the destructor for each owned drop; it does not merge lifetimes by representation equality. Existing resource-handle lowering remains an ownership transfer. Wasmtime corroboration and pinned source citations are in #330.

Host providers that return the same object repeatedly now receive a disposal invocation per independent registration. Plain-object host borrows no longer delay disposal of unrelated own registrations backed by that object. Managing shared backing data is the provider's responsibility.

Verification

• Independent review approved the revised diff.
• Focused embedder/resource/conventions suite: 74 passed, no skips.
• Runtime suite: 865 passed, 3 existing ignored cases; WASI: 293 passed, 1 ignored.
• All just gates recipes completed across resumed invocations: the initial command hit a 120s tool timeout in sched-seeds; remaining recipes resumed with a larger timeout. Installed missing Playwright browsers with just browsers-install, then reran just browsers successfully.
• Seeded runtime/conformance runs, pinned SpiderMonkey/Node and findings-only Bun lanes, Chromium/Firefox page/worker/shared-worker and OPFS checks, npm packaging, Rust/protocol/ct-runner, examples, TLS and C0 consumer smokes passed their configured expectations. JSC shell is skipped on aarch64 by recipe; existing conformance exclusions remain.
• git diff --check, just fmt-check, just lint, just version-guard-local passed. Existing convention goldens unchanged.

Automerge will be armed after creation, with merge after required core checks.