feat: add h/l keys to resize the inspector split from anywhere

[kevinswiber]

Adds h/l keyboard shortcuts to shrink/grow the inspector's split from anywhere — the from-anywhere twin of the divider's focused ArrowLeft/Right, the way j/k step the selection. Closes #413.

Behavior (the three design calls, owner-confirmed)

• h shrinks the timeline pane; past the 25% floor it snaps into reading mode (the divider ArrowLeft twin), leaving the stored ratio intact so a restore returns to the last good ratio.
• l grows it; from reading mode it instead restores the split — the affordance the collapsed divider can't offer, since it's unreachable while reading.
• Step size is the divider's own ~24px-equivalent (reuses stepPct).

How

• New exported stepSplit(dir) in split.ts reuses the divider's currentPct/stepPct/setPct and the single applySplit writer, keeping aria-valuenow in sync so the focused divider and the keys can't disagree. Gated on an open detail pane (a closed pane is a no-op, not an invisible pref write).
• keyboard.ts wires h/l in the main switch after the isTypingTarget yield; preventDefault fires only when stepSplit consumes the key.
• Surfaced in the help sheet (both assets/index.html and the test/support/dom.ts mirror) and as Shrink timeline pane / Grow timeline pane command-palette actions.

Verification

• Strict TDD (red → green): new cases in split.test.ts, keyboard.test.ts, palette.test.ts.
• Front-end gate (Biome + tsc --noEmit + 630/630 vitest, includes bundle-parity) and just check green.
• Live real-key verification in Chrome against a running shore inspect (synthetic events can't reproduce native default-action behavior): l grew 50→60%, h past the floor snapped to reading with the stored ratio held at 26%, l restored to 26%, typing hl into the search box did not resize, and the URL hash never changed (nothing spurious activated).

Reviewed via a Codex Shoreline loop: assessment accepted on agent:codex-413, no open input requests.