engine: de-hardcode the known table and extract world synthesis into nexum-world

[mfw78]

Delete the baked venue rows from the KNOWN capability table so per-namespace rows come from registered extensions, and factor world synthesis into a new plain nexum-world library. The capability model and per-component world synthesis already shipped, but the KNOWN table still bakes a pool row (world.rs:74) and a cow-api to shepherd:cow row (world.rs:80), which leaks a downstream name into the host.

Why

As long as the KNOWN table hardcodes a pool row and a cow-api mapping, the host layer keeps knowledge of specific venues and cow. Sourcing per-namespace rows from registered extensions removes that knowledge, and extracting the synthesis and table into their own library gives the host a clean, venue-free capability core. Part of milestone M2: Generic venue-agnostic host. Blocked by: Grow the Extension seam to carry worker/provider roles. See docs/design/videre-split-plan.md and docs/design/issue-milestone-plan.md.

Scope

• Delete the baked pool and cow-api to shepherd:cow rows from the KNOWN table.
• Source per-namespace capability rows from registered extensions.
• Extract the world.rs synthesis and the KNOWN table into a new plain library, nexum-world.
• Rewrite find_wit_root from a workspace-ancestor walk to crate-local wit/ plus wit/deps resolution.
• Keep #[module] in nexum-module-macros; leave venue and intent macros to videre-macros.

Done when

• The baked pool and cow-api/shepherd:cow rows are removed from the KNOWN table.
• Capability rows are registry-driven from registered extensions.
• World synthesis and the table are extracted into a plain nexum-world library.
• find_wit_root resolves crate-local wit/.
• No venue or cow string remains in nexum-world.

[mfw78]

Landed via #440 (squash-merged into dev/m1). Closing manually since the merge target is not the default branch.