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feat(poxel): tile_interval enclosure and the five #1637 ubit applications (#1641)

* feat(poxel): tile_interval enclosure and the five #1637 ubit applications

tile_interval<Tile> is a guaranteed enclosure built from a pair of ubit
tiles. It works for poxel and for areal tiles of up to 64 bits. Every
operation is evaluated on exact endpoints only. Both tile types return
the tile containing the exact result of an exact-operand op, and that
one tile gives both directed roundings. Tiles are integer keys in tile
order. Open ends stay open, so (0, minpos) + (0, minpos) is strictly
positive. sign() is positive, negative, zero or undecidable, never
wrong. cos is enclosed by the Taylor sums S_30 <= cos <= S_28.

Tests: exhaustive enclosure and tightness for every pair of tiles of
poxel<8,0>, poxel<9,2> and areal<8,2>. Multi-tile operands, open ends,
and cos up to poxel<64,2>. Mutations caught: a shifted bound, missing
product corners, swapped Taylor bounds, endpoint and strictness errors.

The applications compare float, double, posit, single tiles (a flag)
and tile intervals (an enclosure), each with assertions:
- Rump: every rounding format is wrong, posit<32,2> with the wrong
  sign; the intervals contain -0.8274 and are honestly undecidable.
- Muller-Kahan: everything rounding settles on 100; the intervals
  contain the exact iterate at every step.
- det(M^k): double returns 0 (fused multiply-add) or the wrong sign
  (unfused); no interval asserts a wrong sign; 64-bit ones prove
  det(M) > 0.
- BBP tail (reformulated at 16 bits): half flushes to 0, posit<16,2>
  rounds up to minpos; tile_interval<poxel<17,2>> proves
  0 < S < 1e-16.
- Griewank: float, double and posit report g < 0;
  tile_interval<poxel<64,2>> proves g > 0.

The poxel doc's subtraction wording is corrected: a - b adds the
negated tile.

Refs #1637, #1631

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(poxel): check all four k = 50 verdicts; disjoint intersect is nan

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