fix: SSR loader cache isolation and form-submission action race

[uhyo]

Fixes the two high-severity bugs found in the library audit (the remaining findings were filed as issues #204–#219).

1. SSR/SSG loader cache leaked data across renders

Without a navigation entry, loader results were cached under the literal "ssr" key in the module-level cache, so a long-running SSR server (or a multi-page SSG build) served the first render's loader data to every subsequent render — rendering /a then /b in one process showed /a's data on the /b page, and /b's loader never ran.

Fix: executeLoaders gained an optional cache parameter. When the Router has no real location entry (SSR, or Navigation API unavailable), it passes a per-instance cache (created with useState, garbage-collected with the instance) instead of the shared module-level map. Each SSR request/SSG page now executes loaders fresh, while results remain memoized within a single render lifecycle. Client-side behavior with real entry keys is unchanged.

2. Form submissions rendered pre-action loader data

For intercepted form submissions, the entry commits (and currententrychange fires) before the intercept handler runs the action. The Router's transition render executed loaders eagerly with actionResult: undefined, and because the handler's post-action re-execution produced no new snapshot reference, the revalidated data was never rendered: the UI kept the pre-action results, and loader side effects ran twice. (Only visible with actions slower than a microtask, which is why existing adapter-level tests didn't catch it.)

Fix: the adapter creates a pending deferred when it intercepts a form submission and attaches it to the entry-change notification as wait; the Router waits on it inside an async transition (old UI and isPending stay up) before reading the snapshot and rendering. Loaders execute exactly once, with the action result, and the committed render always shows post-action data. The deferred is released on action errors and superseding navigations so a waiting render can never hang.

Testing

• 6 new regression tests: ssrLoaderCache.test.tsx (cross-instance isolation, per-request re-execution, within-instance memoization) and actionRevalidation.test.tsx (slow action, immediate action, action error — driven through <Router> with the browser's real event ordering: navigate → commit → handler → navigatesuccess).
• Full suite: 281 tests + type tests pass; lint, format, and the router package build are clean.
• Note: funstack-router-docs SSG build fails in my container on every commit including master (Node 22 lacks global URLPattern) — unrelated to this change.

Also includes a small docs addition to the Form Actions page describing the (now-guaranteed) pending behavior during submissions.

🤖 Generated with Claude Code

https://claude.ai/code/session_012B1oYTQh1Q7hhBgirpGiek

[uhyo]

Use Promise.withResolvers here.

[uhyo]

This comment is correct, but there is no known plan of supporting transition type addition. So I'd rather write code that works currently.

[uhyo]

This setLocationEntry call is after the await, so won't be included in the transition. Please include it. This can be achieved by a nested startTransition call.

[uhyo]

Let's simply bump ES2022. EESNext is fine.