Open confirmed links and paths in a floating side panel beside the terminal

[jhen0409]

What

Closes #58. A confirmed path or web link now opens in a floating side panel beside the live terminal — a shadowed card over the pane on iPad, a slab hanging off the glass's trailing edge on Vision Pro — instead of as a tab. The terminal is never re-laid out. ↗ TAB moves the same live viewer into a tab; a second summon replaces the first (one panel per terminal tab); the panel closes with its tab and rides along through merge/split; its width is remembered per device. iPhone is unchanged.

Summary

• Ownership: TerminalWorkspace.sidePanels keyed by the HOST terminal tab id (survives merge/split); SidePanelViewController wraps the existing ViewportPaneViewController / FileViewerPaneViewController with one 30-pt action row; AuxiliaryPaneController.routeMode lets ↗ TAB re-register the live controller as an auxiliary tab without a downcast.
• visionOS: a third .trailing UIHostingOrnament hosting ONE static mount — a transparent strip twice the window's width, centred on the glass edge — so a drag never changes the ornament. The card's geometry is stored glass-relative (width, overhang = right edge past the glass, default 440, max 800); both handles have two-way travel from the default and the left handle can pull the card across the terminal down to a 320-pt reserve. Drags freeze the card into one blurred bitmap until release (per-tick reflow and per-tick system-glass resizes were both unusable on device); under GLASS the platter steps out for the drag.
• iPad / Shell: in-window overlay, admitted at ≥ 660 pt pane width in a regular size class; the leading seam resizes live (320 … pane − 320); a Stage Manager shrink below the floor moves the panel into a tab.
• Row: ✕ · ↗ TAB lead, then the viewer's identity and controls, which EXPAND as the width allows by measured priority (▤ TREE · REFRESH · SOURCE|DIFF · SELECT · A− A+ · host; ⌗ SYSTEM); ⋯ only for what did not fit. Header on iPad, bottom bar on visionOS.
• Pure model (SidePanel.swift): admission policy, clamps, strip/overlay geometry and the drag arithmetic, all unit-tested without UIKit. SidePanelWidthStore persists per platform.
• DEBUG hooks debug.sidepanelsplit|sidepanelclose|sidepanelwidth; MULTIPLEX_SIDE_PANEL=0 forces the legacy tab road. 18 new localized keys (zh-Hant, ja); the catalog also gains the 22 iPad-only keys an IDE build had left untranslated.
• Docs: links-and-viewers.md, input-and-windows.md, architecture.md, e2e-headless.md, i18n.md; DESIGN.md, README, TestFlight line.

Proof

Ten feedback rounds on Vision Pro hardware drove the handle design (reach, two-way travel, performance, row placement, control order). Headless E2E on both simulators from a fresh install: ⌗ panel → width hook → ▤ replaces ⌗ → ✕ → ▤ → ↗ TAB → relaunch restores nothing; the visionOS card's default edges measured at the terminal's edge / 440 pt past the glass, the width hook moving only the left edge.

./Tools/build.sh lint clean · visionOS 1328 XCTests green · iPad 1334 green · ruby Tools/check-metadata.rb OK.

Checklist

• ./Tools/build.sh lint passes
• Unit tests pass (MultiplexTests)
• User-visible change noted in fastlane/testflight-whats-new.txt