nav: don't stack a history entry for the place you're already standing on

Walking back with Esc ended in a dead keypress: nav_depth dropped but the screen
didn't change. Measured on a live pane, four Escs after one follow:

  Esc #1 -> sub_3500 L7      (where we followed from)   good
  Esc #2 -> main L54 decomp  (where we followed from)   good
  Esc #3 -> main L344 listing                           good
  Esc #4 -> main L344 listing  -- nav 2 -> 1, nothing moved

The app auto-lands on main at startup, and opening main again from Ctrl+N
appended a second, identical entry. Every "navigate to where you already are"
did this; the extra Esc it bought is invisible except that it does nothing, which
is exactly what "back is broken" feels like from the keyboard.

Both push sites now go through _push_nav, which replaces the top entry instead of
appending when the target is the same (ea, view, line) — the newer entry's
metadata still wins. _same_spot compares dec_cursor for pseudocode and cursor for
the listing.

Note what is NOT a duplicate: the first follow after Tab-to-pseudocode still
pushes twice (nav 1 -> 3). That's deliberate — Tab leaves _cur transient and off
the stack, so the follow records the pseudocode position first, which is why Esc
#1 above returns to main L54 rather than dropping you into the listing. Verified
that subsequent follows push exactly one each.

tests: the palette scenario re-opens the function it is already on and asserts
nav depth is unchanged. Fails without the fix (nav 2 -> 3). 194/0.

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