feat: expose linked TinyBus module entry point

[senamakel]

Summary

• Add the opt-in linked feature and expose tinydocs_module::linked_module() for direct host registration.
• Pin the TinyBus submodule to the module export fix and use the =0.1.2 registry identity for TinyBus dependencies. A local Cargo patch keeps standalone builds on the pinned checkout.

Related issue

Depends on tinyhumansai/tinybus#29. This PR is a draft because its gitlink points to commits from that unmerged PR. Mark ready after #29 merges and the gitlink is reachable from upstream TinyBus, then confirm CI is green.

API or behavior changes

The module package gains linked_module() when built with --features linked. The default loadable module behavior is unchanged.

Validation

• cargo fmt --manifest-path Cargo.toml --all -- --check
• cargo check --manifest-path Cargo.toml -p tinydocs-module --features linked --offline
• Full clippy, build, test, and coverage suites were not run locally.

Tests

The linked feature compile check covers the public reexport and TinyBus macro expansion. No runtime behavior changed in this package.

Documentation

The public API is described here; TinyBus documents the linked host path in #29.

Checklist

• The change is focused on linked module registration.
• No lints were relaxed.
• No secrets or environment contents are included.

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[senamakel]

Superseded by #18, which now carries the static-link entry point, TinyBus registry identity, and updated gitlink.