feat: add Shift+wheel horizontal scroll for wide tables

[aydiler]

Summary

PR #23 removed forward_wheel_to_horizontal_scroll to fix #22 (cursor-over-table nudging wide tables sideways during normal document scrolling). That also reverted the issue #4 ergonomic — wide-table horizontal scroll regressed to "grab the bottom scrollbar" only.

This change reintroduces the wheel→x redirect but gates it on the Shift modifier, so:

• Plain wheel stays with the outer document scroller (Tables scroll slightly horizontally as you scroll past them. #22 stays fixed).
• Shift+wheel over a wide table scrolls the table sideways (Feature requests: Search function and table resizing #4 ergonomic restored as an opt-in).

Shift matches the browser convention for horizontal scroll and doesn't collide with anything else in the doc scroller (Ctrl is already taken by zoom).

Changes

• New helper forward_shift_wheel_to_horizontal_scroll in crates/egui_commonmark/.../pulldown.rs — identical edge-passthrough logic to the prior helper, plus a modifiers.shift early-return.
• Wired into both the markdown-table and HTML-table call sites by capturing the ScrollAreaOutput.
• Docs: KEYBOARD_SHORTCUTS, ARCHITECTURE, TARGET_METRICS, LESSONS, devlog 033.

Test plan

• cargo fmt --check — clean
• cargo check — clean (two pre-existing warnings unrelated to this change)
• cargo clippy --all-targets — clean (same pre-existing warnings)
• Manual: cursor-over-table + plain wheel scrolls the document past the table without horizontal nudge.
• Manual: cursor-over-table + Shift+wheel scrolls the table sideways; edge pass-through hands the wheel back to the document at table edges.
• Manual: Shift+wheel with cursor NOT over a table behaves normally (no-op for tables).

Note: the egui MCP bridge has no keystroke/modifier injection so Shift+wheel behavior can't be E2E-tested via Xvfb. Manual verification on real display required before merge.

Refs #4, #22, #23.