Each live ToolExecutionComponent advanced its spinner glyph from its own
per-instance start time, so concurrent tool spinners showed different
frames and read as janky. Derive the glyph from a single shared monotonic
clock (sharedSpinnerFrame) so every live block animates in lockstep; the
per-instance interval still drives requestRender. The unused
#lastSpinnerAdvanceAt anchor is removed; the todo-strike counter is
untouched.
Adds a `symbols.spinnerFrames` field to the custom theme JSON schema so themes can override the loader/tool-execution spinner that drives `theme.spinnerFrames` / `theme.getSpinnerFrames(type)`. Accepts either a flat `string[]` (used for both spinner types) or an object `{ status?, activity? }` to override each type independently; anything not specified falls back to the active symbol preset (unicode/nerd/ascii). The override is plumbed through `createTheme` and the `Theme` constructor, normalized via `normalizeSpinnerFramesOverride`, and read in `getSpinnerFrames`.
Mirrors the field in `theme-schema.json`, documents it in `docs/theme.md`, and adds a test covering flat-array, per-type, validation-rejection, and preset-fallback paths.
Fixes#1553
metaphorics