Composer position: Flow

[raiseCatError]

Closes #132

Behavior

• Setting: Config → Composer position → Bottom / Top / Flow. The palette's Toggle composer position cycles all three. Older configs load unchanged.
• Following: the composer sits directly after the newest output (after the breathing-space row, or the live activity row while a command runs) and moves down as output grows. Once output fills the screen it lands where Bottom docks it. The PTY always gets the full capacity, so output growth never resizes the shell.
• Scrolling back: the prompt and input are part of NMSh's document and scroll with it. A few rows back, they move down and are clipped at the screen edge. A page back, they are off-screen and the cursor is hidden. There's no switch to the host's scrollback.
• Returning: in Flow, typing, paste, delete, complete, history search or Enter while scrolled back returns to the newest output first, and the key still applies. Wheel, paging and mouse movement alone don't. Bottom and Top are unchanged.
• Placement rules: suggestion and completion menus open below the input. Full-width panels pin to the bottom edge, as with Bottom.
• Chat: the live composer renders normally, and historical commands render right-aligned.
• Passthrough: stays raw. Returning repaints the document and composer from the plan.

Design

• planScreen gains a Flow stack.
• ScreenPlan.viewportRows is the height the history viewport resolves and scrolls by. It's the transcript height for Bottom and Top, and the following capacity in Flow. Without it, a small scroll-back would grow the transcript region into the composer's rows and immediately snap back to the bottom.
• planFrame resolves Flow's scroll position against that capacity and plans from viewStart. Every viewport resolve/scroll site uses viewportRows, while slicing uses the region height.

Decisions (flagged in planning)

• Flow is a third Composer position value rather than a separate Flow/Classic switch, because Dock Top (Composer position: Dock Top #123) already exists.
• Reattach-on-typing applies to Flow only.

Tests

• tests/flowComposer.test.ts: plan placement and growth, menus below, activity, a scrolled-back document (nudged and paged) with fixed PTY/viewport rows, panels, and narrow terminals with long input across follow and scrolled-back states.
• App-level render, cursor and PTY rows; wheel/mouse vs typing; Bottom/Top unchanged; Flow + Chat.
• A real-PTY end-to-end fullscreen passthrough round trip.
• Build, typecheck and git diff --check are clean. The full suite passed 580/580 in the last two local runs.

Limitations

• Flow has no Jump to bottom row while scrolled back. The composer scrolling into view, typing, or the existing latest key returns.
• Multi-line input scrolled partly off screen shows its first visible rows. The caret may be off-screen until you return.
• Physical validation in Ghostty, Terminal.app and VS Code is deferred to the v0.6 test pass.