Add ruleset packs, linter, fixtures, and JSON schema

rulesets/decode.toml

@@ -0,0 +1,19 @@
+[[rule]]
+id = "INVALID_DATA_FOUND"
+domain = "decode"
+severity = "high"
+confidence = 0.7
+fix_tier = "reencode"
+stop_scan = false
+patterns = ["(?i)Invalid data found when processing input"]
+notes = "Decoder encountered invalid data; may indicate corruption."
+
+[[rule]]
+id = "FILE_ENDED_PREMATURELY"
+domain = "decode"
+severity = "high"
+confidence = 0.7
+fix_tier = "reencode"
+stop_scan = false
+patterns = ["(?i)File ended prematurely"]
+notes = "File appears truncated."