Register Mech tokens through the shared spawn path

[abueide]

Problem

The rules engine creates Mechs through a handwritten fallback but does not register Mech as a canonical token name. Three token-name diagnostics are ignored for this known gap. Ferrous Forerunner's existing fallback already creates its printed tokens; this is a registry/integration fix, not a claim that Deathknell previously did nothing.

Change

• Add canonical Token::Mech, domainless 3-Might face and shared name registration.
• Route Mech creation through the common spawn path; retain owner/controller, exhausted entry, token cleanup and Played events.
• Preserve Rumble's public MECH_TOKEN alias and update other Mech consumers.
• Enable three previously ignored diagnostics and test both Ferrous Mechs triggering Rengar independently.
• Token has no serialized discriminant contract; no wire/blob/snapshot format changes.

Validation

Passed in both repositories:

• Focused Ferrous tests including ignored cases.
• Full turns library: Agni 4,581 passed / 169 ignored; agni-rfb 4,567 passed / 169 ignored.
• cargo fmt --all -- --check, pinned Nix treefmt --ci, bash ci/check.sh.

The portable Wasm artifact was not built by this task; no GUI/network playtest claimed. The remaining ignored tests are not counted as verified.

Integration

Companion PR mirrors the still-duplicated rules source; Kai currently consumes Agni. The app dependency/module must be updated to deliver the fix to players.

[abueide]

Mirrored rules PR: abysl/agni-rfb#3 . Kai integration will pin the combined audit fixes; no main branch has been merged by this task.