Help registry drift: 8 normal-mode keybindings missing from :help

[garritfra]

Problem

AGENTS.md states every keybinding MUST have a HelpEntry in crates/cell-sheet-core/src/help/entries.rs — :help lies to users when this drifts. There is no automated check enforcing it; eight implemented Normal-mode keys currently have no help entry:

Key	Implementation	Action
*	mode/normal.rs:498–500	Search current cell value forward
#	mode/normal.rs:502–504	Search current cell value backward
{	mode/normal.rs:418–420	Block jump up
}	mode/normal.rs:422–424	Block jump down
Ctrl+O	mode/normal.rs:160	Jump back
Tab	mode/normal.rs:506–508	Jump forward
V	mode/normal.rs:438–440	Enter Visual Line mode
c (Normal) + c (Visual at mode/visual.rs:83–86)	—	Change

Proposed fix

Two PRs:

1. Add the missing HelpEntry rows to NORMAL_ENTRIES (and Visual c to VISUAL_ENTRIES) in entries.rs.
2. Add an automated drift check so this can never silently regress. Sketch:

// crates/cell-sheet-tui/src/mode/normal.rs
#[cfg(test)]
pub(crate) const IMPLEMENTED_KEYS: &[&str] = &[/* every dispatched key */];

// help/entries.rs test mod
#[test]
fn every_normal_key_is_documented() {
    for k in IMPLEMENTED_KEYS {
        assert!(NORMAL_ENTRIES.iter().any(|e| e.key == *k), "missing help: {k}");
    }
}

Repeat for Insert / Visual / Command / Formula categories.

Priority

HIGH — explicitly flagged in AGENTS.md as a regression-prone seam.