venue: bind Venue::ID to the adapter manifest name

[mfw78]

From review of #460 (r3615872544). Confirmed still valid at the end-of-train tip. Target: videre-macros / videre-sdk (L2) and cow-venue (L3).

A venue's registered id has two unenforced sources:

• shepherd/crates/cow-venue/module.toml declares [module] name = "cow", and its own comment states "The manifest name is the venue id the registry installs". #[videre_sdk::venue] reads this manifest.
• shepherd/crates/cow-venue/src/client.rs hand-types the same id: impl Venue for CowVenue { const ID: VenueId = VenueId::from_static("cow"); }.

Nothing ties the two. Rename [module] name, or register a second deployment under a different id, and CowVenue::ID still compiles while routing to an id that no longer resolves. The failure is a runtime routing fault, not a compile error. The Venue trait doc already states ID is "the id the venue's adapter registers under", so the invariant is documented but unenforced.

Accuracy note on the review's framing: at the tip both sources live in the same crate (shepherd/crates/cow-venue/), in adjacent files, not "a completely separate crate". That narrows the drift window but does not close it, because there is still no compile-time link.

Options

• Cheapest, keeps the current API: have #[videre_sdk::venue] emit a compile-time assertion that the crate's Venue::ID equals the manifest-derived name. Drift becomes a compile error and Venue::ID stays explicit and greppable.
• Stronger: have the macro emit the id const (or a marker type) that the keeper side imports, so the manifest is the single source and nothing is hand-typed.

Timing

Prefer pre-cleave. The fix spans videre-macros/videre-sdk (L2) and cow-venue (L3), so it is one monorepo PR now versus a coordinated cross-repo change after the split.

Acceptance criteria

• A mismatch between a venue adapter's [module] name and its crate's Venue::ID fails at compile time, or Venue::ID is generated from the manifest.
• No hand-typed venue id can silently diverge from the id its adapter registers under.

[mfw78]

Decision (mfw78, 2026-07-22): macro-generated const, not a compile-time assertion.

The venue macro emits the id from the manifest so nothing is hand-typed. This is the stronger option and is effort/days; label updated.

Still gated on #467 (creates crates/cow-venue/module.toml). Note #469 adds module.load.toml and module.sepolia.toml, so generation must cope with more than one manifest per crate, and must satisfy the existing videre-sdk/tests/adapter.rs cases ("demo", "nowhere").