fix: eliminate terminal activation replay and light-theme GPU filter (Mac perf)

[parsakhaz]

Summary

Fixes the multi-second freezes when switching panes/tabs on macOS and the high GPU-helper energy impact (300–400 in Activity Monitor). Root causes found by investigation: every terminal activation replayed its full scrollback through xterm's parser twice (avg 585KB, max 4.8MB per panel measured in a live DB), and light themes ran a per-frame invert(1) hue-rotate(180deg) compositor filter over every terminal canvas.

Work Completed

Mac terminal performance (tmp/done-plans/2026-07-06-mac-terminal-performance.md)

• Activation depth by cause (builds on fix: repaint terminal panels on window refocus (#306) #308's unified activation effect): performance mode does a masked fit+repaint on tab activation and a silent repaint on pure refocus — no reset(), no scrollback replay. Battery saver keeps exactly one full re-sync (its PTY output is cadence-gated while hidden, so the buffer can genuinely be stale). The manual Refresh button keeps full replay.
• WebGL post-attach repaint deferred past next paint (double rAF): refreshing in the same task as the addon attach could hit an uncomposited canvas on blur→refocus reattach, leaving stale pixels.
• Baked inverted light palette: :root.light now defines real --color-terminal-* tokens computed as the exact invert+hue-rotate of the dark palette, so ANSI output looks identical to the old filtered rendering with zero per-frame GPU cost; the .light .xterm filter rule is deleted. Truecolor TUI content now shows true colors (standard emulator behavior). minimumContrastRatio 4.5 in light mode.
• IPC payload cuts: terminal:getState no longer ships the up-to-8MB serialized snapshot alongside raw scrollback, and restore payloads are ceilinged at 512KiB (matches Orca's TERMINAL_SCROLLBACK_REPLAY_BYTE_LIMIT and Superset's MAX_HISTORY_SCROLLBACK_BYTES).
• Serialize-on-hide throttled to once per 10s per panel (snapshot is only consulted on app restart).
• Animations pause on window blur via a window-blurred root class wired to window:focus-changed, not just document.hidden (which rarely fires for a visible-but-unfocused window on macOS).

Pre-Merge Testing

• Switch terminal tabs rapidly in performance mode — instant, brief "Refreshing terminal…" mask, no grid-refit jank, no stale content
• Blur the app 15+ seconds (WebGL detaches), refocus, revisit tabs — no black/blank canvas, no stale pixels
• Battery saver mode: hide window, generate output, re-show — content catches up with one full re-sync
• Light / light-rounded theme: shell ANSI output looks identical to pre-change; selection highlight visible; TUIs show true colors
• TUI (htop/claude) running during tab switches and window resizes — repaints correctly
• Quit and relaunch — terminals restore scrollback with the [Session Restored] marker

Build Verification

• pnpm typecheck passes (all workspaces)
• pnpm lint passes (0 errors)
• pnpm build:main + pnpm build:frontend pass (xterm request-mode build verification OK)