re: recover the IDXD record-key / field-tag hash (8643/8643)

Closes the 4-byte record key. tag_hash is name_hash's shape -- byte-sum
checksum in the top byte over a 24-bit modular polynomial -- with two different
constants: modulus 0x00FFFFDF (2^24-33, prime) instead of 0x00FFF9D7, and no
lowercasing, so tags are case-sensitive. name_hash explains 0 of 8643.

Recovered from the tables rather than the executable: every inline field name
is a known (name -> tag) pair, and comparing names differing in one character
gives the per-position weights 1, 0x100, 0x10000, 0x21, 0x2100, ... -- a byte
leaving bit 24 re-enters as 33, i.e. reduction mod 2^24-33. Holds where it is
easy to get wrong (distance 8 and 9 carry correctly).

A record's key is the tag of its own name: FormationSet rosters 362/362,
UnitGroup rosters 281/281, S02 squadron names 111/111 -- so records can be
addressed by name without reading the roster first.

Implemented in Python (unitgroup.tag_hash) and Rust
(sylpheed_formats::hash::tag_hash) with 3 new unit tests carrying disc-derived
vectors; cargo test -p sylpheed-formats --lib hash is 8/8 green.

Not settled: the guest routine is unlocated, so this uses exact modular
arithmetic where the game may use a Barrett step without final fixup.

docs/re/data/idxd-tag-hash.txt

@@ -0,0 +1,22 @@
+# IDXD record-key / field-tag hash — verification over the whole pak
+
+tag_hash(s) = ((sum(bytes) & 0xff) << 24) | (base-256 polynomial mod 0x00FFFFDF)
+
+named fields with a unique tag in GP_MAIN_GAME_E.pak : 8643
+tag_hash exact                                       : 8643
+wrong                                                : 0
+name_hash exact (the OTHER hash, for contrast)       : 0
+
+sample (name -> tag):
+  AAGunShellID                                 31dd1a13
+  AA_AxisMode_Max                              80b0d604
+  AA_AxisMode_Min                              7eb0ddfa
+  AA_PitchMinus_Max                            6a133241
+  AA_PitchMinus_Min                            68133a37
+  AA_PitchPlus_Max                             028ab283
+  AA_PitchPlus_Min                             008aba79
+  AA_Roll_Max                                  ff8d842e
+  AA_Roll_Min                                  fd8d8c24
+  AA_Yaw_Max                                   97182874
+  AA_Yaw_Min                                   9518306a
+  AB_AA_PitchMinus                             c78e8fdd