promote bcd → strict bitexact (2-digit packed instance, 3 witnesses) (#1319)

* feat(conformance): promote bcd -> strict bitexact (2-digit packed instance)

BCD was structural (generic variable-width, u4_per_digit, no single layout).
This pack fixes the concrete 2-digit packed instance (8-bit: tens[7:4]*10 +
ones[3:0], range 0..99) - the same instance decoded on AX7203 silicon
(corona-decode-bcd, HW Tier-E 100/100) and iverilog-witnessed on bcd_decode.v.

Vectors: 100 valid codes 00..99 (exhaustive over the valid range), abs_error=0.
Three independent witnesses recorded:
  1. Python golden (tens*10+ones, integer exact) - 100/100 abs_error=0
  2. iverilog 13.0 sim of bcd_decode.v - 256/256 bit-exact (incl. invalid-nibble
     mod-128 + valid flag), independent of the Python oracle
  3. FPGA HW (corona-decode-bcd, AX7203 IDCODE 0x13636093) - 100/100 @160000

INDEX: bitexact_packs 69 -> 70, structural_packs 14 -> 13.
Generic-BCD variable-width nature noted in catalog.instance + format_notes;
invalid nibbles (0xA..0xF) excluded from the round-trip claim (documented policy).

Per recipe-sw-bitexact gate (independent decoder + exact oracle + 2nd witness).
Status tag: [verified SW] (iverilog) + [izmereno na kremnii] (HW Tier-E).
Catalog = 83 unchanged.

Closes #1321

* fix(conformance): ASCII-only bcd pack + refresh INDEX sha + NOW sync

- Replace em-dash (U+2014) with ASCII hyphen in bcd_conformance_v0.json
  (fixes integrity-gate Check D parse error: ascii codec byte 0xe2).
- Refresh bcd sha256 in INDEX_all_formats.json to match file byte-for-byte
  (fixes integrity-gate Check C sha_drift).
- Add docs/NOW.md entry for the bcd promotion (fixes check-now-freshness).

No vector values changed; 100/100 abs_error=0 unchanged. Catalog = 83.

Closes #1321

---------

Co-authored-by: gHashTag <admin@t27.ai>

Author: gHashTag

conformance/vectors/INDEX_all_formats.json

@@ -5,9 +5,9 @@
   "preprint": "https://arxiv.org/abs/2606.05017",
   "total_formats": 83,
   "total_packs": 83,
-  "bitexact_packs": 69,
+  "bitexact_packs": 70,
   "selfconsistent_packs": 0,
-  "structural_packs": 14,
+  "structural_packs": 13,
   "packs": [
     {
       "id": "binary16",
@@ -513,10 +513,10 @@
     {
       "id": "bcd",
       "file": "bcd_conformance_v0.json",
-      "kind": "structural",
-      "n_vectors": 0,
-      "source": "generated by gen_all_formats.py",
-      "sha256": "42de461e6878d7a8b891f72b07c9158ac4cc1c92c5dbfbdbbe0c46c60429dd29"
+      "kind": "bitexact",
+      "n_vectors": 100,
+      "source": "2-digit packed BCD instance; iverilog-witnessed (256/256) + HW Tier-E",
+      "sha256": "637912a0b8d1a4a1e5a50ddafa2c6dbc8e44c7e96218fb8c70763bc69de505cb"
     },
     {
       "id": "ibm_hfp32",
@@ -663,4 +663,4 @@
       "sha256": "fc2a0a6dcce7bbb0eccc1e23ebdacb9abbd81cd54111796be6dc7e6a87a2071a"
     }
   ]
-}
+}