Add semantic micro-motion to application rail icons

[Nocehi]

Summary

Give the rice-owned application-rail commands semantic hover/focus choreography while preserving the current 56px rail topology, command behavior, and browser-module boundary.

Base: main@a9534cc3368f9485cb4a0ed6d84882c2386788c8.

Physical dogfood corrections

Four local dogfood findings shaped this version:

1. scale-based end states made the controls read primarily as the icon getting larger;
2. 140ms one-shot nudges made Sessions/New Session read as a twitch, while Activity was effectively imperceptible;
3. the first Scan/Activity pass still behaved like playback rather than interaction: Sessions launched a complete autonomous scanner animation on hover, and Activity incorrectly removed the baseline together with the shell;
4. the adapted Scan inherited the old magnifier's 22px box despite having a larger optical footprint, while New Session still carried a known-suspect placeholder nudge.

Current product behavior

• Sessions: the command is still identified from the existing MATERIAL_SYMBOL_PATHS.search binding, but the rice-owned rail presentation renders an adapted Carbon ScanMotion glyph at an explicit 20px inside the unchanged 36px optical seat. Hover/focus transitions the scanner frame and three scan bars into a held active pose with a short stagger. Leaving reverses that stagger from the current rendered state; there is no autonomous loop and no need to wait for a full animation cycle.
• New Session / add: the existing Google Material Symbol stays 24px and static. The provisional clipped-part 1px nudge is removed entirely; choosing a later compose/new-session grammar remains a separate design decision.
• Activity / browse_activity: the existing Google path is split into upper shell, lower shell, baseline, and waveform. The baseline and waveform are the persistent signal layer and remain visible throughout. Hover/focus wipes only the lower shell and then upper shell; the held state preserves baseline + waveform, and release restores upper then lower shell.
• Big Fat Whale stays passive and stable.

The rail remains 56px wide with the same 36px optical seats, semantic seat hover/pressed/focus states, badges, commands, and adaptive coarse-pointer targets.

Motion / accessibility boundary

Pointer hover choreography is enabled only for a fine pointer with hover capability and prefers-reduced-motion: no-preference.

Keyboard :focus-visible reaches the same held states. Reduced-motion users receive neither the scanner transitions nor the Activity shell transitions. New Session is static regardless of motion preference.

Provenance / dependency boundary

Google Material Symbols Rounded remains the source for add, browse_activity, and the underlying search command identity used to recognize the Sessions rail control.

The rendered Sessions scanner geometry and scanner/bar transform relationship are adapted from Carbon Icon Animations ScanMotion, Apache-2.0. THIRD_PARTY_NOTICES.md records the donor files and IBM copyright; the adapted source carries an explicit modification notice. The donor geometry is retained, while dsh-rice changes output size to 20px and replaces the old autonomous 2s keyframe envelope with interruptible hover/focus transitions and reverse release behavior.

No @carbon/icons-motion, motion package, Framer Motion, or other browser dependency is added. The browser artifact still requires only React plus @deepseek-ai/dsh-client-ui-primitives.

Verification

Focused artifact coverage asserts:

• Sessions is still bound to the existing search command but renders the adapted scanner geometry at 20px;
• scanner enter/release is transition-driven, staggered, held, and non-looping;
• release ordering is the reverse of enter and can return from an in-progress state;
• the old magnifier handle nudge and autonomous Scan keyframes are absent;
• New Session falls back to the ordinary static 24px Material Symbol path, with no rail add motion classes or keyframes left;
• Activity renders upper shell, lower shell, baseline, and waveform separately;
• only the two shell paths participate in the directional wipe;
• baseline and waveform have no motion rule and remain visible during hover/focus and release;
• hover/focus capability gates and reduced-motion boundaries remain in place;
• no Carbon runtime or new motion/icon dependency is introduced.

docs/rail-motion.md records the dogfood history, sizing decision, interaction contract, representation boundary, provenance, and accessibility contract.