feat(app): replace wave loading indicator with spherical-harmonic morphing dot #261

Acceptance Criteria

• When a turn is streaming, the tail step's rail dot renders as a ~13px SVG that morphs through four spherical-harmonic silhouettes on a ~2.3s hold per mode
• The morph uses SMIL path interpolation — all four paths have identical point counts (64 samples)
• The dot slowly rotates (~12s per full turn) via CSS transform on a parent <g>
• The dot grows from 7px (done size) to ~13px with a smooth CSS transition (~150ms ease-out) when it enters the running state
• When the step completes, the dot shrinks back to 7px ink circle (the existing done styling)
• The ThinkingLine no longer renders the wave SVG or cycling gerund — only the meta row (elapsed, tokens, esc hint)
• Under prefers-reduced-motion: no morph animation, no rotation; the running dot renders as a static 13px yellow circle
• The morphing dot fits within the existing pl-6 (24px) content inset — no layout shift to surrounding content
• Both light and dark themes render correctly (accent fill is visible on both grounds; accent-edge border defines the shape on light where yellow alone is ~1.2:1 contrast)

Testing Decisions

• Visual verification in the running app (localhost:4444 dev server) across light/dark themes
• Verify prefers-reduced-motion via DevTools emulation → Rendering → Emulate CSS media feature
• Confirm no layout shift with the browser's Layout Shift debugger or by observing content reflow
• Unit-testable: the geometry module (harmonic-geometry.ts) should be testable standalone — correct point counts, normalized radii, valid SVG path syntax

Key Decisions

• SVG path morph via SMIL — all four harmonic outlines are sampled at 64 angular points (equal M/L command counts), enabling native <animate attributeName="d"> interpolation without JS frame loops or third-party libraries
• 13px running size — centered on LINE_X (11px), extends 4.5–17.5px, stays inside the 24px content gutter with no gutter-width changes needed
• Slow rotation on parent <g> — CSS transform: rotate() on the <g> wrapper, separate from the SMIL d morph on the <path>, so the two animations compose cleanly
• Keep wave files — amico-wave.tsx and wave-geometry.ts stay in the tree (potential reuse on marketing surfaces) but are no longer imported by ThinkingLine
• Meta line retained as-is — elapsed time, token count, and interrupt hint continue rendering in their current DOM position below the message; only the wave+word above them is removed

Constraints & Invariants

• The rail's geometry rules (documented in thought-rail.tsx header) remain intact — dot centre alignment, segment cap logic, adjacency-based completion
• Done dots remain 7px ink circles — the morph is exclusively a running-state behavior
• The NODE constant (7px) stays for done-state rendering; a new RUNNING_NODE (~13px) applies only inside HarmonicDot
• No new runtime dependencies — pure SVG + CSS + SMIL, framework-agnostic geometry module
• The ThinkingLine's metadata row must not reflow or jump when the wave is removed (the grid layout simplifies but the meta's position is stable)

Prior Art

• packages/ui/src/amicode/wave-geometry.ts — the standing-wave approach (modal paths, timing constants, pure geometry module pattern)
• packages/app/src/pages/session/timeline/thought-rail.tsx — the rail's six geometry rules, especially Rule 5 (gutter sizing) and Rule 6 (lone running dot)
• packages/ui/src/amicode/amico-wave.tsx — SMIL-compatible multi-mode SVG animation pattern

Notes

• The four harmonic modes chosen (Y_0^0, Y_1^0, Y_2^0, Y_2^2) are recognizable at small sizes: circle, peanut, elongated pinch, and four-lobe clover. Higher harmonics (l=3+) have too many lobes to resolve at 13px.
• The mode cadence (2.3s) matches the wave's existing MODE_HOLD_MS, maintaining the same temporal rhythm the eye is already trained on.
• The grow transition (7→13px) serves double duty: it signals "this step just started" and it gives the morph enough pixels to be legible.