[Epic] Touch/pointer input support

[stormmuller]

Goal

Add touch/pointer input support. src/input currently only has keyboard, mouse, and gamepad input sources (src/input/{keyboard,mouse,gamepad}) — there is no touch or unified pointer-event handling, so games can't currently support mobile/touch devices.

Scope (suggested)

• A new src/input/touch (or unified pointer) input source module, following the existing input-source pattern used by keyboard/mouse/gamepad.
• Multi-touch support (tracking multiple simultaneous touch points, not just a single pointer), since this is a common need for mobile games (virtual joysticks, pinch-to-zoom, multi-finger gestures).
• Consider unifying mouse + touch behind a common "pointer" abstraction where it makes sense (e.g. tap/click, drag), while still exposing raw touch data for multi-touch-specific use cases.
• Camera pan/zoom (src/rendering's camera system) is a natural first consumer to validate the API against, given it already supports mouse-driven pan/zoom.

Why this is a milestone, not a single issue

This is a new input-source module plus integration points across existing systems (camera controls, any future UI system), and needs e2e coverage (per AGENTS.md's e2e testing conventions) similar to the existing mouse-input.spec.ts/keyboard-input.spec.ts.

Acceptance criteria (top-level)

• Touch input is captured as a first-class input source, supporting multi-touch
• Covered by unit tests and an e2e spec (e2e/specs)
• Documented in documentation-site/docs/docs/input