refactor(navigation): hierarchical, route-dispatching CodeNavigator

[bmc08gt]

Summary

Simplifies the "too many navigators inside a flow" problem. Inside a FlowHost a screen previously had to reason about several navigator handles (inner flow navigator, outer/app navigator, sheet navigator, FlowNavigator), and LocalCodeNavigator.current silently resolved to different instances depending on context. This makes CodeNavigator hierarchical and route-type-dispatching so a screen reaches every scope through the one LocalCodeNavigator it holds — a FlowStep stays on the flow stack, anything else bubbles up the parent chain to the app-root navigator.

Rebased onto current code/cash.

Phases (staged, each behind the previous)

Phase 1 — engine (behind existing API, no behavior change)

• CodeNavigator gains parent, isFlowNavigator, and a FlowScope; navigate() dispatches by route type.
• New navigateBackWithResult() and dismiss() intents (delegating to FlowScope).
• FlowHost wires the inner navigator with parent + FlowScope; sheet-dismiss guarded on isSheetRoot.

Phase 2 — FlowNavigator thin adapter

• InnerFlowNavigator reimplemented as a thin adapter delegating back/navigate/exitWithResult to the engine; redundant outerNavigator field removed. Behavior-preserving.

Phase 3 — call-site migration (kill the context-ambiguity)

• Chat, deposit, withdrawal, contact-list, and swap flows migrated to a single dispatching LocalCodeNavigator, removing silent flowAnnotatedEntry / CompositionLocalProvider re-shadows and collapsing redundant handles.

Phase 4 — eliminate LocalOuterCodeNavigator

• Two engine primitives: dispatchTarget(route) (the navigator a route lands on) and rootNavigator (the sheet-hosting root).
• navigateForResult registers its callback on dispatchTarget(route) — so a cross-flow-boundary result nav works from the inner navigator (the callback lands on the store the returning screen delivers to). Intra-flow FlowStep results are unchanged (dispatchTarget returns this).
• openAsSheet operates on rootNavigator — so it works from a flow's inner navigator. Backward-compatible: existing root callers get rootNavigator === this.
• Swap and Login migrated to LocalCodeNavigator; dead flowAnnotatedEntry and LocalOuterCodeNavigator deleted. LocalSheetNavigator intentionally retained (chat's legitimate sheet-host handle).

Also included

• fix(chat): eliminate message-list reflow on open and back-navigation. The conversation screen rebuilt from zero-initialized layout state on every open/pop. ChatInputScaffold now uses SubcomposeLayout (correct list padding on frame 1), refreshSettled is rememberSaveable, and OnChatOpened skips a redundant reload on re-entry (live delivery via the app-scoped event stream keeps Room current, so no messages are missed).

Testing

• :ui:navigation:testDebugUnitTest, :apps:flipcash:shared:router:testDebugUnitTest green; full assembleDebug green.
• In-app verified: chat send/back flow + reflow fix.
• In-app checks still to run on this build: (1) swap → verification resumes after verifying (cross-boundary result delivery); (2) login → Lab opens as a bottom sheet (openAsSheet via rootNavigator).