runtime: bring http egress under the compile-time world guarantee

[mfw78]

Bring venue http egress under the compile-time synthesised-world guarantee and canonicalise a single adapter import-narrowing contract, retiring the blanket shim path.

Why

Two coupled gaps let egress escape the capability model. First, http escapes the compile-time guarantee: in the KNOWN table http has import: None (world.rs:88-92), so wasi:http is linked out-of-band and gated only by the engine.toml allowlist; the undeclared-capability-is-a-compile-error guarantee therefore covers chain, messaging, and logging but not http. Second, there are two adapter contracts: export_venue_adapter! imports chain and messaging unconditionally and leans on wasm-tools dead-import elision, while synthesize_venue narrows by construction. Either bring http under the synthesised-world guarantee or document loudly at the seam that http egress is allowlist-gated, and settle on one import-narrowing contract. Part of milestone M7: Egress guard. Blocked by: egress-guard-hardening-epic. See docs/design/videre-split-plan.md and docs/design/issue-milestone-plan.md.

Scope

• Bring http under the synthesised-world guarantee, or document at the seam that http egress is allowlist-gated only.
• Canonicalise one adapter contract that narrows imports by construction.
• Retire the blanket shim path that imports chain and messaging unconditionally and relies on dead-import elision.

Done when

• Undeclared http egress is a build error, or the allowlist-only story is documented at the seam.
• Exactly one adapter contract exists, with import narrowing by construction.