Migrate the composed smokes to a polymorph:test conformance suite

[lann]

Adopts the polymorph:test harness for cross-implementation conformance, replacing the smoke-tls / smoke-tls-delegated / smoke-tls-webcrypto rigs. One decision — the composed smokes become the shared suite — with its consequences; the pieces are co-dependent (deleting the loopback crates while keeping their recipes, or landing the suite while keeping the duplicate smokes, leaves main worse between PRs).

What lands

• conformance/guest-ct — the suite: the loopback smokes recast as discrete cases (identity/ed25519/reject-foreign-key, handshake/alpn-sni, data/echo, shutdown/close-notify, delegated/*) on the polymorph:test contract, the polymorph:tls surface imported exactly as a consumer would. tests.lock is the committed inventory.
• conformance/driver-ct — targets, recipes, committed matrix. A target is a composition: the suite ⊕ one delivery, so the generic ct-runner executes every target (composed, the artifact imports only wasi 0.2 + test-context — no custom driver crate, unlike the webcrypto sibling which has a host-side SUT). Standing targets: composed (in-guest Ed25519 posture) and composed-delegated (⊕ fixture signer); composed-delegated-webcrypto (⊕ webcrypto-signer shim ⊕ real polymorph:webcrypto provider) is on demand and optional, replacing smoke-tls-webcrypto.
• Feature model: one gated feature, delegated-signer. The old structural gate ("delegated identity must fail without a composed signer") becomes the !delegated-signer decline case — the decline pair the harness's tag scheme calls for.
• CI: a conformance job (just conformance), the aggregate action pinned at the workspace rev with a skew grep guard, ct-tools cached by Cargo.lock rev, results artifact. The smoke job drops to smoke-quic + smoke-tls-virt.
• The import-satisfaction and signer-reachability grep gates from the old recipes carry over into conformance-ct::build.

What deliberately doesn't

• jco targets — blocked on two upstream jco defects in the composed p3 stream/future path (TDZ emission order for the future.transfer trampoline; the callee task's teardown clearing the current-task register before the return-position streamTransfer of a fused sync call), reproduced against lann/jco jco-transpile-0.6.2-fa6990d. Tracked with repro in Conformance: jco targets blocked on upstream jco defects in composed p3 stream/future paths #28; filed upstream as -I async: future.transfer trampoline const is emitted after the init generator that references it (TDZ ReferenceError at instantiation) lann/jco#34 and Fused sync call returning a stream: callee task teardown clears CURRENT_TASK_META before the return-position streamTransfer ('missing global current task') lann/jco#35.
• In-suite QUIC cases (in-memory noq loopback, RFC 9001 vectors per target) — a coverage extension, not part of this cutover: Conformance: in-suite QUIC cases (in-memory noq loopback, RFC 9001 vectors on every target) #29.
• smoke-quic, interop, bench, audit, the timing lab, and the tls-virt rigs stay as they are (conformance/README.md records the boundary).

Verification

• just conformance: 7 cases; composed 5 pass + 2 N/A, composed-delegated 6 pass + 1 N/A; lock-check and matrix-check green.
• just conformance-ct::run-webcrypto + aggregate: 21 results, 0 failing across all three targets (delegated cases pass over the real webcrypto provider).
• just check (incl. the new wasm32 clippy pass over the suite) and just smoke-quic green.

Incidental fix folded in where the recipe moved: the sibling checkout under target/webcrypto-src is a build cache, so checkout -f — a previously dirtied lockfile wedged the old recipe on rev moves.