sdk: make the venue macro the single blessed authoring path

[mfw78]

Make #[videre::venue] the single blessed venue authoring path, fixed to emit impl VenueAdapter. Two authoring paths currently fork the one arrangement.

Why

Today #[venue] emits impl Guest over raw bindgen and bypasses the typed VenueAdapter trait, while export_venue_adapter! routes through it on a differently-named world that imports chain and messaging unconditionally. Two paths for one job is a fork; collapsing to one blessed macro that always emits impl VenueAdapter removes the ambiguity and the hand-copied bridge boilerplate. Part of milestone M3: Videre SDK, macros and DX. Blocked by: videre-sdk: rename nexum-venue-sdk + add Keeper::sweep assembler + VenueClient. See docs/design/videre-split-plan.md and docs/design/issue-milestone-plan.md.

Scope

• Fix #[videre::venue] to emit impl VenueAdapter, the videre:venue/adapter export and the manifest kind, not a raw Guest impl.
• Demote export_venue_adapter! to the internal codegen the macro expands to, so there is no public second path.
• Narrow imports by construction via the synthesized venue world, rather than dead-import elision.
• Remove the roughly 80-line *_to_golden bridges; port echo-venue to the macro.
• Land the macro in videre-macros after the macro split.

Done when

• #[videre::venue] emits impl VenueAdapter plus the videre:venue/adapter export and the manifest kind, not a raw Guest impl.
• export_venue_adapter! is demoted to the internal codegen the macro expands to, with no public second path.
• Import narrowing is by construction, with no dead-import elision.
• echo-venue uses the macro and the *_to_golden bridge boilerplate is gone.

[mfw78]

Landed via #458 (squash-merged into dev/m1 as 3765ecf). Closing manually since the merge target is not the default branch.