refactor(reader): drop redundant offset > fileSize bounds clause

Mutation testing flagged the `offset > fileSize` comparison in both
validateSegmentSpecs and checkBlobBounds as an equivalent mutant — changing
its boundary never changed an outcome. It is genuinely dead: once `length < 0`
is excluded, length >= 0, so an offset past fileSize makes fileSize - offset
negative and the final `length > fileSize - offset` clause already fires. The
clause can never independently decide the result.

This is the same overflow-safe shape as IoBounds.checkRange (which omits the
clause); the parser had hand-rolled the guard and drifted from its own
primitive. Removing it simplifies both predicates and eliminates the
equivalent mutants rather than papering over them with unkillable-by-design
tests. Existing bounds tests stay green and keep their context-named messages.

Reader mutations 110 -> 106 (4 dead mutants gone), killed 90%.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dfa1

reader/src/main/java/io/github/dfa1/vortex/reader/PostscriptParser.java

@@ -78,7 +78,10 @@ static void validateSegmentSpecs(List<SegmentSpec> specs, long fileSize) {
             SegmentSpec s = specs.get(i);
             long offset = s.offset();
             long length = s.length();
-            if (offset < 0 || length < 0 || offset > fileSize || length > fileSize - offset) {
+            // Overflow-safe containment in [0, fileSize], same shape as IoBounds.checkRange. An
+            // `offset > fileSize` clause would be redundant: with length >= 0 already guaranteed,
+            // offset > fileSize forces length > fileSize - offset, so the final clause covers it.
+            if (offset < 0 || length < 0 || length > fileSize - offset) {
                 throw new VortexException(
                         "footer segmentSpecs[" + i + "] out of bounds: offset=" + offset
                                 + " length=" + length + " fileSize=" + fileSize);
@@ -87,7 +90,10 @@ static void validateSegmentSpecs(List<SegmentSpec> specs, long fileSize) {
     }
 
     private static void checkBlobBounds(String name, long offset, long length, long fileSize) {
-        if (offset < 0 || length < 0 || offset > fileSize || length > fileSize - offset) {
+        // Same overflow-safe range form as IoBounds.checkRange (no redundant `offset > fileSize`
+        // clause: length >= 0 makes it implied by the final comparison). Keeps the blob-named
+        // message that checkRange's generic text would lose.
+        if (offset < 0 || length < 0 || length > fileSize - offset) {
             throw new VortexException(
                     "postscript " + name + " blob out of bounds: offset=" + offset
                             + " length=" + length + " fileSize=" + fileSize);