SBlk head characterised: four constants and one word that is not a count

sound_bank's head was parsed but never characterised. Across all 27 levels the
SBlk region opens identically for its first twelve bytes:

    +0x00 "SBlk"   +0x04 1   +0x08 4   +0x0c "DAW\0"   +0x10 0

and +0x14 is the only word that varies. It varies with its low sixteen bits zero
on all 27, leaving 0x0092..0x0103 -- 146 to 259.

That is the shape of a record count, and it tracks the body length at close to
100 bytes per unit. It is not one:

    body / field_14     91.2 .. 110.8 across 27 levels
    any stride 0..80 with body = stride*word + constant exactly:  no

So the test asserts the negative too. If a stride ever makes the relation exact
on all 27, sound_bank_sblk_head_is_constant_apart_from_one_word fails and asks
for the record size to be revisited. A near-miss at 100 bytes/record is exactly
what hardens into a name the next time somebody glances at it; the constants are
also checked individually so a real deviation names which one moved.

The DAW region begins with sixteen zero bytes then block-structured data with
16-byte periodicity and slowly varying nibbles -- consistent with block
compressed audio, not decoded. Recorded as such.

Two mistakes, both mine, both the same shape as earlier ones:

1. The constructor bound `format_tag` with a `let` BEFORE the two words that
   precede it in the file, consuming bytes 4..8 and shifting every field after
   it. The struct literal listed the names in the right order; the statements did
   not. A scratch script reading the same bytes gave the opposite answer, which
   is what exposed it. Fixed, with a comment, because "read the lets in file
   order" gets undone otherwise.

2. Inserting the new test ahead of an existing one stranded that test's #[test]
   on my doc comment -- silently unregistering
   tie_vertex_element_is_16_bytes_with_fixed_texcoords. This is the SAME mistake
   as two commits ago and I made it again. It again surfaced only as clippy's
   "never used" rather than as a missing test. Restored and verified by listing
   the registered tests rather than trusting the count.

240 tests, clippy clean, both tie vertex tests confirmed present.

docs/formats/core-data-regions.md

@@ -2142,3 +2142,45 @@ field means anything. The test's power came from `i16[0..2]` being the only slot
 with tens of thousands of distinct values. **A passing result from a method that
 has run out of discriminative power is not a passing result**, and the 1x rows are
 recorded so the table is not later mistaken for nine confirmations.
+
+## The `SBlk` head: four constants, one unidentified word
+
+The sound extent's 24-byte head was parsed but not characterised. Across all 27
+levels the `SBlk` region opens identically for its first twelve bytes:
+
+    +0x00  "SBlk"
+    +0x04  1
+    +0x08  4
+    +0x0c  "DAW\0"
+    +0x10  0
+    +0x14  varies
+
+`+0x14` is the only part that differs between levels, and it differs with its
+**low sixteen bits zero on all 27**, leaving `0x0092..=0x0103` -- 146 to 259.
+
+That is the shape of a record count, and it tracks the `SBlk` body length at
+close to 100 bytes per unit. It is not one:
+
+    body / field_14      91.2 .. 110.8 across the 27 levels
+    any stride 0..80 making body = stride * word + constant exactly:  no
+
+A near-miss at 100 bytes/record is exactly the kind of thing that hardens into a
+name the next time somebody glances at it, so the test asserts the negative
+alongside the positive: if a stride ever makes the relation exact on all 27, the
+test fails and asks for the record size to be revisited.
+
+Asserted in `sound_bank_sblk_head_is_constant_apart_from_one_word`, which checks
+the four constants individually so a real deviation names which moved, the zero
+low half, the value range, and the absence of a fixed stride.
+
+The `DAW` region begins with sixteen zero bytes and then block-structured data
+with 16-byte periodicity and slowly varying nibbles, consistent with block
+compressed audio. It is not decoded.
+
+Two notes on how this was measured. Reading the level 0 head directly from a
+scratch script gave a field order that the parser disagreed with, because the
+constructor bound `format_tag` with a `let` *before* the two words that precede
+it in the file -- consuming bytes 4..8 and shifting everything after. The struct
+literal named the fields in the right order; the statements did not. It is fixed
+with a comment saying so, since "read the `let`s in file order" is the kind of
+thing that gets undone.

docs/roadmap.md

@@ -228,7 +228,11 @@ Phase 1 hands over the extents; this phase works out what each one contains.
       with the WAD LZ77 decoder, bit order and header length ruled out" note
       was wrong about the cause: the decoder was right and the input was not
       what it looked like. Corrected in `plan/level-assets.md`.
-- [~] `sound_bank` — container framing parsed (`SoundBankHeader::parse`,
+- [~] `sound_bank` — the `SBlk` head is now characterised: `"SBlk", 1, 4,
+      "DAW\0", 0` constant on 27/27, and `+0x14` is the only varying word, with
+      its low 16 bits always zero (`0x0092..=0x0103`). It tracks body length at
+      ~100 bytes/unit and is **not** a count -- ratio 91.2..110.8, and no stride
+      makes it exact. The absence of a stride is asserted too. Container framing parsed (`SoundBankHeader::parse`,
       `parse_sound_bank`); `SBlk` and `DAW` bounds verified on 27/27. The
       **record fields inside `SBlk` are not decoded**, and `DAW` shows no
       detectable stride, consistent with ADPCM. See the note under