arrowUp icon key carries upload, send, and directional meanings

## Problem

The built-in `arrowUp` icon name currently carries three different semantic responsibilities:

| Consumer | Use | Meaning |
| --- | --- | --- |
| `FileInput` | upload affordance | action: upload |
| `ChatSendButton` | default send glyph | action: send |
| sortable `Table` | ascending indicator | direction: ascending |

The directional meaning is coherent with `arrowDown` and `arrowsUpDown`. The upload and send meanings only happen to share the same artwork today.

This becomes observable when a theme customizes the registry. Mapping `arrowUp` to an upload-tray glyph also changes the chat send button and breaks the visual pairing between the table's ascending and descending indicators. A single registry key therefore cannot be customized predictably across its current callsites.

## Current architecture

The current icon architecture separates shared semantic names from component-owned roles. That makes the boundary important:

- `arrowUp` should remain a directional name.
- Upload and send are reusable UI actions, not FileInput- or Chat-specific implementation details.
- Component-owned slot mappings can still be added later if a theme needs to remap one component role independently; that is a separate layer from the shared semantic vocabulary.

## Proposed contract

Add two shared semantic names:

- `upload`
- `send`

Both would initially resolve to the existing upward-arrow artwork, so default rendering would not change. FileInput would use `upload`, ChatSendButton would use `send`, and sortable Table would keep `arrowUp`.

This lets a theme provide an upload-tray glyph without changing send or sort, and a send glyph without changing upload or sort.

## Alternatives considered

### Keep `arrowUp` for all three meanings

No source change, but registry customization remains coupled across unrelated actions and direction.

### Add only component-owned slots

Slots are useful when one component role needs a different shared meaning. They do not by themselves provide shared `upload` or `send` meanings for themes to map to appropriate artwork. These two actions are expected to recur across components, so they belong in the shared semantic vocabulary first.

## Scope and non-goals

- No default visual change.
- No change to `arrowDown` or `arrowsUpDown`; their current uses remain directional.
- No component-slot implementation in this proposal.
- No commitment to a specific upload or send drawing; themes continue to own artwork.

## Suggested sequence

1. Settle the shared-name contract in a spec-only pull request.
2. Add the two names and default artwork aliases.
3. Migrate FileInput and ChatSendButton to the new names with focused registry tests and consumer documentation.