Replace opacity pulse with sprite-based working animation in catwalk

[williamthorsen]

Problem

The catwalk uses an opacity sine-wave pulse (0.7–1.0 at 1.5 Hz) to indicate agents in the working state. This reads poorly — it looks like a rendering glitch. The problem is compounded because opacity is already used for state differentiation:

• idle: 0.3 / resting: 0.6 / deactivated: 0.15 / active: 1.0
• working pulse minimum: 0.7 (barely distinguishable from resting at 0.6)
• orchestrator pulse minimum: 0.6 (identical to resting)

Opacity is doing double duty: state identity and activity signaling. These are orthogonal concerns that shouldn't share the same visual channel.

Context

• PR #223 Create sprite-loading infrastructure for catwalk #228 introduced sprite-loading infrastructure with placeholder SVG sprite sheets
• The working state cycles through three sprite frames (S4→S5→S6 at 300ms), but with current placeholders these frames are visually indistinguishable
• Issue Create pixel art for catwalk visualization #222 will eventually provide real pixel art with distinct working poses
• Issue Generate block-robot pixel art sprites programmatically #235 will provide programmatic block-robot sprites as an interim improvement

Solution

Replace the opacity pulse with a scale pulse — a subtle "breathing" effect (1.0→~1.08 at 1.5 Hz) that signals activity without conflicting with opacity-based state differentiation.

• Working agents render at full opacity (1.0) with a gentle scale oscillation
• Both StationAgentActor and OrchestratorActor share the same scale pulse range (unlike opacity, scale is purely an activity signal and doesn't need per-actor tuning)
• Scale resets to 1.0 when leaving the working state
• Opacity-based state differentiation (idle 0.3, resting 0.6, deactivated 0.15) remains unchanged

Acceptance criteria

• Opacity pulse removed from StationAgentActor and OrchestratorActor during working state
• Working agents render at full opacity (1.0)
• Scale pulse (1.0→~1.08 at 1.5 Hz) replaces opacity pulse as activity indicator
• Both actor types share the same scale pulse constants
• Scale resets to vec(1, 1) when leaving working state
• Existing opacity-based state differentiation (idle 0.3, resting 0.6, deactivated 0.15) unchanged
• Unused opacity pulse constants removed from animation.ts
• All existing tests pass; pulse-related tests updated to verify scale behavior
• Typecheck, lint, test all pass