Graph: fix the triskel fallbacks, and say why when it still falls back

"engine triskel -> native+triskel-failed" was unreadable and, worse,
gave no reason: the cause went to a logger a TUI user never sees.

Dumped all 400 functions of bin/ls (echo's 128 were not enough) and swept
them at three zoom levels: 6 of 1200 layouts fell back, all of them my
own _verify tripping over a detour that could not be placed.

  - the detour jumped to the nearest side of the FIRST box in the way,
    which in a dense layout is usually inside the next box along. It now
    collects every box the run passes and picks the nearest genuinely
    free line.
  - it skipped the first and last segments because they carry the port
    and the arrowhead. But that is exactly where the failures were:
    triskel is happy to park a block directly above its successor and
    drive the final approach straight through it. Those segments may now
    move ALONG their own box's border, which is free almost every time.
  - repairs are swept to a fixed point: moving one segment stretches its
    neighbours, which can push those into a box.

0/1200 fallbacks after that. Then the bigger corpus turned up a second,
genuinely upstream defect: superimposing SESE regions can leave two
blocks a couple of columns into each other (2 of ls's 400 functions,
in float space, before rounding). Cosmetic in a PNG; here the boxes are
made of text, so one block's disassembly overwrites another's. _verify
now checks it and falls back, which is the right trade.

Reporting, so this is never mute again:
  - stats["engine_error"] carries the reason, the status line shows it,
    and the label is "native (triskel failed)".
  - the corpus test asserts fallbacks are rare AND explained, rather
    than asserting they never happen.

Also lowered AUTO_TRISKEL_MAX_BLOCKS 250 -> 180. Layout runs on every
zoom keypress and triskel knees hard past ~175 blocks (174: 66ms,
233: 489ms, 329: 555ms). The old cap allowed a 489ms stall. The corpus
timing check now measures only sizes `auto` can actually reach, plus a
5s ceiling so nothing blows up quadratically when forced.

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