refactor(http): route HTTP bindings through perry-ffi

[TheHypnoo]

Summary

• Route production perry-ext-http runtime calls through narrow perry-ffi operations.
• Remove the normal perry-ext-http -> perry-runtime edge and its staticlib workaround.
• Keep the selected libperry_ext_http.a artifact and exported HTTP ABI unchanged.
• Add the required changelog fragment in changelog.d/6834-http-ffi-boundary.md.

Proof

• cargo tree -p perry-ext-http --edges normal has no perry-runtime.
• Production HTTP code has no perry_runtime:: references; explicit test shims retain the test-only dev dependency.
• Release export comparison: 215 -> 215 relevant HTTP symbols, empty diff.
• cargo test -p perry-ffi --features runtime-link: 49 passed.
• cargo test -p perry-ext-http: 68 passed.
• Architecture self-test/check and Cargo metadata passed.

Scope

This stacked PR excludes Phase 4 stdlib HTTP deduplication and unrelated cleanup.

Known CI limitations

PR #6826 remains blocked by its inherited lint failure (the Phase 2B head has a pre-existing cargo fmt --all -- --check diff in crates/perry-hir/src/lower/builder_fold.rs) and several conformance jobs. A full release binary link-and-run smoke exceeded the local two-minute command limit after repeated incremental builds; the release HTTP archive and 215-symbol comparison passed.

Related issue

N/A — Perry architecture roadmap Phase 3.

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