dx(ui): warn when hydration ends with unclaimed SSR nodes

[viniciusdacal]

Description

When endHydration() is called, if the cursor hasn't reached the end of siblings at any level, the SSR and client trees diverged. The developer gets no feedback about this — extra SSR nodes are silently left in the DOM.

Context

• In packages/ui/src/hydrate/hydration-context.ts, endHydration() (lines 33-37) resets all state without checking if nodes were left unclaimed:

  export function endHydration(): void {
    isHydrating = false;
    currentNode = null;
    cursorStack.length = 0;
  }

• If the SSR output has <div><h1>Title</h1><p>Extra paragraph</p></div> but the client only claims <h1>, the <p> remains in the DOM unclaimed. This is correct behavior for tolerant mode (don't destroy extra content), but the developer should know their trees diverged.
• The cursor stack may also have entries if enterChildren/exitChildren were unbalanced at end of hydration (separate from the empty-stack warning in exitChildren which fires mid-hydration).
• This should be console.debug (not warn) since extra nodes from browser extensions are expected and normal.

Tasks

1. Before resetting state in endHydration(), check if currentNode !== null (unclaimed siblings remain).
2. Check if cursorStack.length > 0 (unbalanced enter/exit).
3. Log appropriate dev-mode debug messages.

Files to modify

• packages/ui/src/hydrate/hydration-context.ts — endHydration()

References

• Parent PR: feat(ui): add tolerant hydration mode for mount() [#510] #619
• Parent issue: feat(ui): tolerant hydration mode for mount() — scoped diff with browser extension tolerance #510
• Design doc: plans/mutable-herding-journal.md (Claude plans)

Acceptance Criteria

• Test: hydration ends with unclaimed sibling → console.debug with message like "[hydrate] Hydration ended with unclaimed nodes remaining. SSR tree may have extra content."
• Test: hydration ends with unbalanced stack → console.debug with stack depth info
• Test: clean hydration (all claimed, stack empty) → no debug output
• Zero overhead in production builds
• Browser extension nodes triggering this debug message is acceptable and expected

Progress

• 2026-02-22: Created from PR feat(ui): add tolerant hydration mode for mount() [#510] #619 re-review findings