fix(tui): patch nanostores map.setKey to mutate $session in place

[kelsonpw]

Summary

PR #760 added the pendingEnvSelection flag and gated every post-Auth show: predicate on it, but the live bug still reproduced. Real root cause is a stale-reference bug between bin.ts and the WizardStore:

• bin.ts holds a session ref that it mutates in-place during checkpoint hydration, resolveCredentials, and applyEnvSelectionDeferral, then calls tui.store.session = session to re-emit.
• The store's $session = map<WizardSession>(...). Nanostores' map.setKey reassigns $session.value to a fresh { ...prev, [key]: value } object on every key change.
• The first in-store setter (typically concludeIntro fired when the user clicks Resume on the checkpoint prompt while resolveCredentials is still awaiting) detaches $session.value from the bin.ts session ref.
• After that point, bin.ts's later in-place mutations — including the deferral that sets pendingEnvSelection = true and clears credentials — never reach the store.
• The final tui.store.session = session then OVERWRITES the store's accumulated state with the stale bin.ts ref, wiping the user's introConcluded = true progress and producing either a bump back to Intro or a stall with no env-picker surface.

PR #760's flag write site (session.pendingEnvSelection = true) is correct in isolation — it just lands on a session object the store already stopped reading from.

The fix

Patch $session.setKey to mutate $session.value in place instead of allocating a fresh spread. notify() still fires per key, so the React useSyncExternalStore bridge (which reads $version) is unaffected. The wizard's UI doesn't use listenKeys anywhere, so the per-listener oldValue arg semantics change is non-observable.

With the patch, $session.value and the bin.ts session ref point at the same object for the entire run. External mutations propagate transparently; store setters don't clobber them.

Regression test

Added two tests in src/ui/tui/__tests__/flow-invariants.test.ts:

1. preserves the bin.ts session reference after an in-store setKey — pins ref stability across concludeIntro.
2. propagates in-place external mutations after an in-store setKey, including pendingEnvSelection — replays the exact bin.ts sequence: build session → startTUI → concludeIntro → in-place deferral mutations → final store.session = session. Asserts currentScreen === Auth (env picker surface) and that introConcluded survives the resync.

Both tests fail on origin/main (verified by stashing the store change) and pass with this commit.

Test plan

• pnpm exec tsc --noEmit clean
• pnpm exec eslint . clean (one pre-existing warning unrelated to this change)
• pnpm build succeeds (smoke test passes)
• pnpm exec vitest run --pool=forks --maxWorkers=1 --testTimeout=30000 — 3955/3955 passing
• Regression test fails on origin/main without the store change

🤖 Generated with Claude Code

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Note

Medium Risk
Changes core TUI session state update semantics by overriding nanostores map.setKey, which could affect any code relying on object identity or listenKeys old/new value behavior; however the change is scoped to the Wizard session store and covered by new regression tests.

Overview
Fixes a TUI race where WizardStore session updates could detach from the bin.ts session reference, causing in-place mutations (notably pendingEnvSelection) to be missed and later store.session = session re-emits to clobber progress.

WizardStore now builds $session via a custom createSessionStore() that patches nanostores map.setKey to mutate the existing session object in-place while still calling notify().

Adds regression tests in flow-invariants.test.ts to assert (1) session reference stability across concludeIntro() and (2) propagation of external in-place mutations plus preservation of introConcluded and correct routing back to Screen.Auth.

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