fix(core): normalize embedded terminal keys (#1423)

## Summary
- reject parser escape codes such as `[A` and `[27u` as physical key
identifiers
- infer Web-style physical identities (`ArrowUp`, `KeyC`, `Digit1`,
`Escape`) before calling Ghostty
- derive the unshifted codepoint from that normalized physical identity
- cover raw arrow input and nested Kitty character/escape re-encoding

## Why this is needed
OpenTUI key events use `code` for two different kinds of values. Kitty
and raw escape parsing can produce parser codes such as `[A` for Up or
`[27u` for Kitty Escape, while other events provide Web-style physical
codes such as `KeyA`. Ghostty's key encoder accepts the latter physical
identities, not parser escape fragments. Passing `[A` through as the
physical key leaves Ghostty unable to encode an arrow key.

Embedded terminals also need to re-encode input when the program inside
them enables the Kitty keyboard protocol. In that mode, text alone is
insufficient for modified and disambiguated keys: Ghostty needs a
normalized physical identity and its unshifted codepoint. Raw character
events may not include `code`, so this change infers `KeyA`/`Digit1`;
Kitty escape events whose parser code is `[27u` fall back to `Escape`.
The regression verifies that Ctrl+C and Escape are re-emitted as Kitty
sequences for a nested terminal.

## Testing
- `bun test src/renderables/EmbeddedTerminal.test.ts` (16 passed)
- `oxlint packages/core/src/renderables/EmbeddedTerminal.ts
packages/core/src/renderables/EmbeddedTerminal.test.ts`
- `oxfmt --check packages/core/src/renderables/EmbeddedTerminal.ts
packages/core/src/renderables/EmbeddedTerminal.test.ts`