nav: bind back to backspace as well — a bare Esc isn't reliable in a terminal

Esc-to-go-back needed two presses. It looked like the decomp pane was swallowing
the first one, but the RPC state dump before/after a single Esc settles it:

    nav_depth   3  ->  3      action_back never ran
    cursor.col  19 -> 20      +1 column, i.e. `right`
    modal       null          nothing was up to eat the key

The keypress arrived as a right-arrow. \x1b is both Escape and the lead byte of
every arrow/function-key sequence (ESC [ C is right), so a terminal or tmux that
merges a lone Esc with what follows delivers something else entirely. Nothing in
the nav path touches cursor_x and there is no widget-level escape binding on the
code views, so this was never application logic — I spent two commits looking in
the wrong place before asking for the dump.

Bind back to "escape,backspace". Backspace is unambiguous, so going back never
depends on a byte the terminal can reinterpret; Esc still works wherever it's
delivered intact. tmux users generally want `set -sg escape-time 10` regardless.

docs/TEXTUAL_NOTES.md records the symptom and, more usefully, the triage: check
nav_depth in the state dump first — if the action never ran, it's the input
layer, not the logic.

Verified: backspace in the pseudocode pane takes nav 3 -> 2 and reloads the
previous function. Suite 192/0.

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