Add configurable piecewise and bounded operator shortcuts

[Shrookin]

What

Adds configurable piecewise and bounded-operator LaTeX shortcuts while preserving normal MathLive editing behavior:

• \piecewise{n} creates an editable native cases structure with n rows.
• \int{x}{y}, \sum{x}{y}, and \prod{x}{y} create bounded operators.
• Deleting content from a bounded lower or upper argument leaves an editable \placeholder{} instead of removing the bound.
• After the body and both bounds are cleared, Backspace traverses to the operator and removes it cleanly instead of getting stuck at the leading sentinel.
• Text-mode LaTeX commands now wait for completion instead of eagerly committing a prefix. For example, typing \piecewise no longer commits \pi while the command is incomplete.
• Adds the compact virtual-keyboard keycap class for tall structured labels.

Why

MathLive users need parameterized structural shortcuts that remain editable after insertion. Prefix commands must also behave like ordinary LaTeX input: a shared prefix should remain pending until the command is complete or explicitly finalized. Structural deletion must preserve editable placeholders without making an empty operator impossible to remove.

Implementation

• Registers a parameterized piecewise parser that builds the same native cases-array structure used by regular editing.
• Registers bounded parsers for int, sum, and prod while preserving their existing unbounded forms.
• Uses editable bounded argument atoms and deletion handling to retain placeholders when bound contents are removed.
• Removes an empty bounded operator only after its body, lower bound, and upper bound have been traversed.
• Defers the backslash in text mode into LaTeX command mode through the native keybinding definitions.
• Prevents custom structured commands from being serialized as opaque verbatim source.
• Keeps the compact keycap styling generic and independent of inserted LaTeX.

Consumer synchronization

The dependent applications remain compatible with MathLive 0.110 while consuming the shared behavior:

• NDQ: 86eda86 keeps structural LaTeX command caret handling visible and preserves bounded-operator editing through its adapter.
• Skillboard: 976e0582 marks the shared piecewise key as compact while retaining the legacy styling hook.
• Parse Tree: 8edfa46 applies the same keyboard-key compatibility update.

Verification

• npx tsc --noEmit -p tsconfig.json — passed.
• npx jest test/ --runInBand — 344 tests passed.
• Focused Chromium Playwright coverage for piecewise insertion, deferred text-mode commands, bounded placeholder deletion, and full bounded-operator deletion order — passed.
• Focused Chromium Playwright coverage for compact structured virtual-keyboard labels — passed.
• NDQ full suite — 255 tests passed; targeted adapter follow-up — 15 tests passed.
• Skillboard MathLive integration tests — 3 passed; production build passed.
• Parse Tree production build passed.

The branch remains draft while the behavior is reviewed.