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feat(ereal): constants and math library past the 313-digit ceiling (#1576)

* feat(ereal): constants and math library past the 313-digit ceiling

Part 5 of #1355. Two ceilings held every ereal transcendental at ~313
digits, whatever the limb type and however many limbs it was given.

1. The stored constants were plain double-component expansions, so the
   table ended where double's range does: 19 entries. Each component now
   carries its own binary exponent and is rebuilt as ldexp(FpType(m), e),
   so one table serves every limb type and runs to 2^-4298 (79 entries,
   ~1300 digits, emitted by generate_reference_constants.py
   --ereal-limbs). A limb type takes what it can hold: double limbs stop
   at entry 19 -- exactly the old table, so their constants are
   byte-identical -- while x87 and binary128 take all of it. pi with long
   double limbs goes from 313 digits to 1311.

   The narrow path needs care: a float limb cannot hold a 53-bit
   mantissa, so each term is built as a double and split by the
   conversion, and every limb of the split must be normal. Admitting a
   subnormal component there made pi/2 * 2 stop agreeing with pi after 45
   digits.

2. Every series stopped by converting its term to a double and comparing
   against 10^-(53*maxlimbs/3.322). Any term below ~1e-308 reads as zero
   there, so a series stopped at ~308 digits on a limb type whose range
   goes further. The test is on exponents now (series_converged), and the
   iteration counts follow the limb type.

   That also lifts the double results, which had been stopping short of
   what the type holds: at 19 limbs 4*atan(1) goes from 304 digits to 321
   and exp(1) from 305 to 320; at 8 limbs log(2) from 128 to 145.

The constants and all 15 math-function headers are templates on the limb
type. ereal<24, long double> reaches 482 digits of log(2) and 4*atan(1).

BEHAVIOUR-CHANGE: ereal transcendentals carry the digits their configuration holds, where the series stopped ea... (continued)

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