fix(erf): align restype numbers with upstream neverwinter.nim

Five entries in the restype table disagreed with upstream
neverwinter.nim's restype.nim, so HAKs written from those source
extensions were labelled with a type the game reads as something else:

  gff  0x07F7 (2039, upstream bte) -> 0x07F5 (2037)
  ltr  0x0813 (2067, upstream bak) -> 0x07F4 (2036)
  jpg  0x081C (2076, upstream tml) -> 0x0821 (2081)
  dfa                              -> dft (0x07FD/2045 was already right,
                                      only the name was a typo)
  mdb  0x0816 (2070, upstream xbc) -> removed

Upstream registers no restype for mdb, so there is no correct number to
give it; keeping it squatting on xbc silently mislabels the resource.
It is dropped from AssetExtensions too, which turns .mdb into a loud
"unsupported extension" build error instead of a corrupt HAK entry. No
asset in the current corpus uses any of these paths.

Root cause of the miss: the init() consistency guard only panicked when
a number was missing from the reverse map. Its `if canonicalExt == ext
{ continue }` branch did nothing when the two maps disagreed, so a
mismatch was never caught. The guard now panics on disagreement and
also checks the reverse direction.

Adds a conformance test pinning the shared numbers against upstream,
including the four numbers upstream reserves for other extensions, and
a test that the two maps are mutual inverses.

Refs ShadowsOverWestgate/sow-tools#64

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/erf/erf.go

@@ -67,6 +67,10 @@ type resourceEntry struct {
 	Size   uint32
 }
 
+// extensionTypes and typeExtensions must stay exact inverses of each other;
+// init() panics if they drift apart. Numbers below 0x0BB8 follow upstream
+// neverwinter.nim (neverwinter/restype.nim). The 0x0BB8 and up entries are
+// local additions for source formats upstream does not register.
 var extensionTypes = map[string]uint16{
 	"res": 0x0000,
 	"bmp": 0x0001,
@@ -96,12 +100,13 @@ var extensionTypes = map[string]uint16{
 	"utt": 0x07F0,
 	"dds": 0x07F1,
 	"uts": 0x07F3,
+	"ltr": 0x07F4,
+	"gff": 0x07F5,
 	"fac": 0x07F6,
-	"gff": 0x07F7,
 	"ute": 0x07F8,
 	"utd": 0x07FA,
 	"utp": 0x07FC,
-	"dfa": 0x07FD,
+	"dft": 0x07FD,
 	"gic": 0x07FE,
 	"gui": 0x07FF,
 	"utm": 0x0803,
@@ -117,14 +122,12 @@ var extensionTypes = map[string]uint16{
 	"ndb": 0x0810,
 	"ptm": 0x0811,
 	"ptt": 0x0812,
-	"ltr": 0x0813,
 	"shd": 0x0815,
-	"mdb": 0x0816,
 	"mtr": 0x0818,
-	"jpg": 0x081C,
 	"lod": 0x081E,
 	"gif": 0x081F,
 	"png": 0x0820,
+	"jpg": 0x0821,
 	"lyt": 0x0BB8,
 	"vis": 0x0BB9,
 	"mdx": 0x0BC0,
@@ -162,12 +165,13 @@ var typeExtensions = map[uint16]string{
 	0x07F0: "utt",
 	0x07F1: "dds",
 	0x07F3: "uts",
+	0x07F4: "ltr",
+	0x07F5: "gff",
 	0x07F6: "fac",
-	0x07F7: "gff",
 	0x07F8: "ute",
 	0x07FA: "utd",
 	0x07FC: "utp",
-	0x07FD: "dfa",
+	0x07FD: "dft",
 	0x07FE: "gic",
 	0x07FF: "gui",
 	0x0803: "utm",
@@ -183,14 +187,12 @@ var typeExtensions = map[uint16]string{
 	0x0810: "ndb",
 	0x0811: "ptm",
 	0x0812: "ptt",
-	0x0813: "ltr",
 	0x0815: "shd",
-	0x0816: "mdb",
 	0x0818: "mtr",
-	0x081C: "jpg",
 	0x081E: "lod",
 	0x081F: "gif",
 	0x0820: "png",
+	0x0821: "jpg",
 	0x0BB8: "lyt",
 	0x0BB9: "vis",
 	0x0BC0: "mdx",
@@ -205,8 +207,13 @@ func init() {
 		if !ok {
 			panic(fmt.Sprintf("missing canonical extension for resource type 0x%04X", resourceType))
 		}
-		if canonicalExt == ext {
-			continue
+		if canonicalExt != ext {
+			panic(fmt.Sprintf("resource type 0x%04X is %q in extensionTypes but %q in typeExtensions", resourceType, ext, canonicalExt))
+		}
+	}
+	for resourceType, ext := range typeExtensions {
+		if registered, ok := extensionTypes[ext]; !ok || registered != resourceType {
+			panic(fmt.Sprintf("extension %q is 0x%04X in typeExtensions but 0x%04X in extensionTypes (registered=%v)", ext, resourceType, registered, ok))
 		}
 	}
 }