Add boundary-double sweep test cross-checking Żmij against dragonbox

verify.py --dump-boundaries now emits test/boundary-bits.h, the bit
patterns of the rounding-boundary doubles it enumerates. The new
dtoa_test.boundary_sweep formats dragonbox's to_decimal the way Żmij
renders and compares it to dtoa on every such double, so it runs across
all build variants including the C one.

Drop the hand-picked dtoa_test.boundary_cases: seven of its values are
covered by the sweep and the two exact powers of two by all_irregular.
Link the bundled fmt into the test targets for the exponent formatting.

Enhance performance metrics in README