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fix(elreal): e_zbcl full-precision terms + exact constant oracle (#1048) (#1054)

* fix(elreal): e_zbcl full-precision terms + exact constant oracle (#1048)

#1048: validate the elreal constant generators to hundreds of digits against the
320-digit reference strings, using an exact-dyadic oracle that shares no code with
elreal arithmetic. A host-double tolerance check (~16 digits) cannot see
high-precision operator bugs; an exact comparison can.

Oracle (include/sw/universal/verification/elreal_reference_digits.hpp): converts a
ZBCL to its exact dyadic value (einteger-backed dyadic_exact.hpp) and reports the
number of leading significant decimal digits it agrees with a decimal reference
string, by cross-multiplied einteger comparison (no rounding).

It immediately caught two bugs the double-tolerance test missed:

1) e_zbcl summed host-double 1.0/n! terms -- each rounded to ~k bits -- so e was
   correct to only ~17 digits no matter the depth. Fixed: accumulate the
   reciprocal factorial as a ZBCL (term_n = term_{n-1} / n, exact div), so 1/n!
   keeps depth-block precision. e now reaches ~276 digits, matching pi/ln2.

2) euler_gamma_zbcl is a bare double literal (~17 digits) -- a stub with no
   high-precision algorithm. Filed as #1053 and excluded from the deep check.

Validation test (elastic/elreal/math/constants_highprecision.cpp): asserts the
nine constants with a real generator + reference (pi, e, ln2, ln10, log2_10, phi,
sqrt2, sqrt3, sqrt5) agree to >= 270 digits. Measured at depth 20:
pi/e/ln2/ln10/sqrt3 276, log2_10 277, phi/sqrt5 280, sqrt2 282.

The double-host ceiling is ~280 digits today: the generator/mul/div/renorm
pipeline floors at the double normal range (~2^-1022) instead of the block's
symbolic int32 exponent. Lifting that floor to reach 300+ digits is a core-
arithmetic change tracked in #1052; this test asserts the honest current ceiling
and raises the threshold once #1052 lands.

Cost: generating to the ceiling is O(depth^4) (... (continued)

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