arvo: posit number library (lib/unum) - #7374

Add lib/unum.hoon (numerics /lib/unum): Standard-2022 posit arithmetic
(es=2 uniform across widths) -- encode/decode, the elementary ops and
transcendentals, and quire support. Pure Hoon; depends on /+ twoc for
two's-complement helpers.

Add tests/lib/unum-core, unum-fns, unum-edge. Verified: arvo compiles and
all three suites are green on a fresh %zuse-408 ship.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

- H12: document ++fdp silent truncation behavior
- M24-M25: doccord correctness fixes
- L15: add per-arm doccords to seven quire op arms

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

…t hints)

This PR was cut from urbit/numerics before two changes landed there; this
commit mirrors numerics main's lib/unum.hoon and its tests byte-for-byte, per
numerics' merge guide (numerics Hoon is ground truth, mirrored to
pkg/arvo/lib/).

1. Correctly rounded transcendentals (urbit/numerics#71).  The series
   expansions this PR carried were one ulp off for several posit32 results.
   Measured on a ship, the previous head of this branch returned the left-hand
   column; numerics' exact-rational posit encoder (80-digit mpmath) gives the
   right-hand one, which is what the refreshed library returns:

     op          before (series)   correctly rounded (now)
     exp(0.5)    0x4530.94c8       0x4530.94c7
     cos(0.5)    0x3e0a.9404       0x3e0a.9403
     tan(0.5)    0x38bd.a7ad       0x38bd.a7ae
     log(2)      0x3b17.2180       0x3b17.217f

   #71 replaced them with range-reduced Chebyshev approximations and refreshed
   tests/lib/unum-edge and unum-fns to match; both come across here too
   (unum-core is unchanged).

2. Jet hints (urbit/numerics#65, #79).  The library now carries its ~%/~/
   hints, matching the registration in urbit/vere#1046, whose SoftUnum jets are
   already on the same Chebyshev basis -- so the Hoon and the jets agree
   bit-for-bit.  Without #1046 the hints are inert.  lib/math in #7372 ships
   its hints the same way.

Nothing from this PR's previous commits is lost: every line of the audit-fix
commit (d22c333) is present in numerics main.  unum calls only the twoc
door arms gth/lth/gte/lte/msb, all defined in #7373's lib/twoc, and nothing
else in the tree references lib/unum.

Verified on a fresh fakezod (hoon-135/zuse-408) with #7373's lib/twoc and this
tree's lib/test: tests/lib/unum-core 12, unum-edge 28, unum-fns 15 -- all 55
green.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TgBnKsUPYzkoPZonjePnaq