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_test.go

@@ -134,8 +134,7 @@ func TestExtensionMappingsSupportModernAssetTypes(t *testing.T) {
 		"mtr": 0x0818,
 		"shd": 0x0815,
 		"txi": 0x07E6,
-		"jpg": 0x081C,
-		"mdb": 0x0816,
+		"jpg": 0x0821,
 		"lyt": 0x0BB8,
 		"vis": 0x0BB9,
 		"mdx": 0x0BC0,
@@ -191,3 +190,58 @@ func TestExtensionMappingsSupportAllUTBlueprintTypes(t *testing.T) {
 		}
 	}
 }
+
+// TestRestypeTableMatchesUpstream pins the restype numbers Crucible shares with
+// neverwinter.nim's restype.nim. The values below are upstream's; a mismatch
+// means a HAK we write is mislabelled for the game.
+func TestRestypeTableMatchesUpstream(t *testing.T) {
+	upstream := map[string]uint16{
+		"res": 0, "bmp": 1, "mve": 2, "tga": 3, "wav": 4, "plt": 6, "ini": 7,
+		"bmu": 8, "txt": 10, "mdl": 2002, "nss": 2009, "ncs": 2010, "are": 2012,
+		"set": 2013, "ifo": 2014, "bic": 2015, "wok": 2016, "2da": 2017,
+		"tlk": 2018, "txi": 2022, "git": 2023, "uti": 2025, "utc": 2027,
+		"dlg": 2029, "itp": 2030, "utt": 2032, "dds": 2033, "uts": 2035,
+		"ltr": 2036, "gff": 2037, "fac": 2038, "ute": 2040, "utd": 2042,
+		"utp": 2044, "dft": 2045, "gic": 2046, "gui": 2047, "utm": 2051,
+		"dwk": 2052, "pwk": 2053, "utg": 2055, "jrl": 2056, "utw": 2058,
+		"ssf": 2060, "hak": 2061, "nwm": 2062, "bik": 2063, "ndb": 2064,
+		"ptm": 2065, "ptt": 2066, "shd": 2069, "mtr": 2072, "lod": 2078,
+		"gif": 2079, "png": 2080, "jpg": 2081,
+	}
+	for ext, want := range upstream {
+		got, ok := ResourceTypeForExtension(ext)
+		if !ok {
+			t.Errorf("%s: not registered, upstream has %d", ext, want)
+			continue
+		}
+		if got != want {
+			t.Errorf("%s: registered as %d, upstream has %d", ext, got, want)
+		}
+	}
+	// Extensions upstream registers that Crucible must not reuse for anything else.
+	reserved := map[uint16]string{2039: "bte", 2067: "bak", 2070: "xbc", 2076: "tml"}
+	for number, upstreamExt := range reserved {
+		if ext, ok := ExtensionForResourceType(number); ok {
+			t.Errorf("%d: registered as %s, upstream reserves it for %s", number, ext, upstreamExt)
+		}
+	}
+}
+
+// TestRestypeTablesAreMutualInverses is the check init() is meant to enforce.
+func TestRestypeTablesAreMutualInverses(t *testing.T) {
+	for ext, number := range extensionTypes {
+		canonical, ok := typeExtensions[number]
+		if !ok {
+			t.Errorf("%s: number 0x%04X missing from typeExtensions", ext, number)
+			continue
+		}
+		if canonical != ext {
+			t.Errorf("%s: maps to 0x%04X, which maps back to %s", ext, number, canonical)
+		}
+	}
+	for number, ext := range typeExtensions {
+		if got, ok := extensionTypes[ext]; !ok || got != number {
+			t.Errorf("0x%04X: maps to %s, which maps back to 0x%04X (ok=%v)", number, ext, got, ok)
+		}
+	}
+}