fix adapter cancellation ownership races

[momics]

Summary

• replay an early Tauri stopServe() after native serve registration so lifecycle shutdown cannot be lost
• give each pathChanges() iterator an adapter subscription ID and use one token-aware core watcher to fan out per peer
• make unsubscribe and endpoint cleanup ownership-scoped across Node, Deno, and Tauri
• add regressions for pre-registration aborts, overlapping observers, rapid resubscribe, and early Tauri serve shutdown

Why

The exact-SHA extended run for c828112 exposed two bridge scheduling races: Tauri could lose a stop before serve registration, and Node could lose an abort before the native path subscription existed. A first adapter-only ownership pass was rejected during review because core still owned one peer-wide sender. This version fixes ownership at the source while retaining exactly one polling watcher per peer.

Validation

• cargo test -p iroh-http-core --test observe (3/3)
• Node Rust tests (5/5), Deno Rust tests (8/8), Tauri Rust tests (59 + 4 + doc)
• Tauri guest tests (86/86)
• TypeScript typecheck for all workspaces
• strict clippy for core, Node, Deno, and Tauri
• complete rebuilt Node extended suite (88/88, 152s)
• focused rebuilt Node path observer suite (2/2)
• two independent final race reviews: clean