testing: promote shared test helpers into the published test crates

[mfw78]

Follow-up from the end-of-train refactor analysis. Target: cross-repo (L1 nexum-runtime + L2 videre + L3 shepherd). Priority: medium.

Problem

Several test helpers are re-derived per site because downstream repos can only reach nexum helpers through the published test-utils feature and the nexum-sdk-test/videre-test tags, not through #[cfg(test)] items.

1. videre-host reimplements the Supervisor boot ceremony (crates/videre-host/tests/platform.rs:324 calls Supervisor::boot_single; tests/zero_leak.rs:69 calls Supervisor::boot) because nexum-runtime's exported TestRuntime (crates/nexum-runtime/src/test_utils/harness.rs:145, :203) only surfaces logs/chain/store/clock and never exposes the booted Supervisor, which the venue tests need for services().get::<VenueRegistry>(), adapter_alive_count(), and dispatch_block().
2. Manifest TOML is rebuilt ad hoc: harness.rs:322 (block_manifest), harness.rs:340 (chain_log_manifest), and platform.rs:303 (intent_status_manifest), differing only in the subscription stanza.
3. AssetAmount/value-flow constructors are open-coded everywhere: platform.rs:239 (native), crates/videre-test/src/reference.rs:98 (minimal_be, the canonical trim-leading-zeros wire rule, locked private), crates/videre-sdk/tests/adapter.rs:70, crates/videre-test/tests/conformance.rs:35.
4. Two different structs named RecordedRequest are re-exported side by side: the chain mock's (crates/nexum-runtime/src/test_utils/chain.rs:113) and videre-test's HTTP one (crates/videre-test/src/transport.rs:78, re-exported at src/lib.rs).

Change

• Widen the exported harness so extension authors can drive a real boot: add TestRuntime::supervisor(&self) -> &Supervisor<E> (or a boot_supervisor constructor on TestRuntimeBuilder) so videre-host can drop its bespoke engine+linker+boot glue.
• Add a typed manifest builder to test_utils, e.g. manifest(name).require([...]).subscribe_block(chain).subscribe_chain_log(chain).to_toml(), used by both the runtime harness tests and the videre platform tests.
• Promote native(), erc20(token, amount), and minimal_be() into videre-test's public surface (the shared conformance crate, already consumed by cow-venue and echo-venue).
• Rename videre-test's HTTP RecordedRequest to RecordedFetch to end the name collision.

Acceptance criteria

• videre-host's two integration tests boot via the exported harness; Supervisor::boot/boot_single are no longer called directly in test code.
• A single manifest builder replaces block_manifest/chain_log_manifest/intent_status_manifest and the stray inline literals.
• native/erc20/minimal_be are public in videre-test; the inline AssetAmount constructions in the listed test sites use them.
• Only one RecordedRequest type exists; the HTTP one is RecordedFetch.
• All changes stay strict-up through the published crates.

Addendum: test-file hygiene + mock consolidation (from #447 review, 2026-07-21)

Beyond the shared helpers above, the integration test files carry AI-generated cruft to strip as part of this pass:

• Decorative section dividers. crates/videre-host/tests/platform.rs uses box-drawing horizontal rules as section headers - // ── fixtures + assembly ──────, // ── world contract ──────, // ── intent-status subscription E2E ──────, // ── echo round trip ──────, // ── venue-adapter trap recovery ────── (5 in one 714-line file). These are decorative cruft against the terse house style; strip them (a plain lowercase comment or nothing suffices). Sweep the other test files for the same pattern while consolidating.
• Duplicate scripted mock adapters. ScriptedAdapter (platform.rs, a status-script VenueAdapter) duplicates StubAdapter (the registry unit tests' with_status_script mock in videre-host/src/registry.rs). Consolidate into one shared scripted mock adapter in the test-support surface.

The locator / engine / linker / block / workspace_path scaffolding duplication (module_wasm_or_skip x4, make_wasmtime_engine x3, make_linker x3, fn block x5) is tracked in detail in #494 (exported nexum locator + a shared tests/common/mod.rs). Together #494 + #497 should leave the integration suites DRY and cruft-free; a house-style de-cruft sweep of the test files is in scope for whichever lands first.