Return the same Thread object from Thread.currentThread()

[dlunch]

Summary

Thread.currentThread() was a stub that constructed a brand-new Thread object on every call, so currentThread() == currentThread() was false, and inside a started thread currentThread() had no relation to the Thread object that was started.

Changes

• Every attached thread owns a java/lang/Thread instance, stored on the JVM's per-thread state. Jvm::attach_thread now takes the instance for threads started from Java (Thread.start passes its receiver, so currentThread() inside run() is the started object) and creates one otherwise (bootstrap/main thread, external attachers).
• Thread.currentThread() returns the stored instance; missing is an invariant violation.
• The GC treats each thread's Thread object as a root; detach_thread drops it with the thread state.
• Classfile parsing: unrecognized attributes are now kept as an opaque Unknown variant instead of failing the parse — JVMS 4.7.1 requires unrecognized attributes to be silently ignored. Found because the anonymous-class fixture carries EnclosingMethod/Signature attributes the parser rejected outright.

Test plan

• CurrentThread E2E (expected output from a real JVM): same-thread calls compare equal, currentThread() inside a started Runnable is the started Thread object, and differs from the main thread's. Fails before the change (false/false/false); the anonymous-class fixture also failed to parse before the attribute fix.
• cargo test --workspace green, fmt clean, clippy no new warnings.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.08%. Comparing base (fe5d116) to head (9ccb383).

Additional details and impacted files

@@ Coverage Diff @@## main #175 +/- ##
==========================================
+ Coverage 83.89% 84.08% +0.19% 
==========================================
Files 173 173 Lines 12939 12958 +19 ==========================================
+ Hits 10855 10896 +41 + Misses 2084 2062 -22 

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[Copilot]

Pull request overview

This PR fixes java.lang.Thread.currentThread() so it returns the same per-attached-thread java/lang/Thread instance instead of allocating a new object on every call, aligning identity semantics with real JVM behavior (including inside started threads). It also updates classfile parsing to tolerate unknown attributes as required by the JVMS.

Changes:

• Store a per-thread java/lang/Thread instance on JvmThread, and make Thread.currentThread() return that stored instance.
• Update thread attach/start paths so Java-started threads attach using the started Thread receiver; bootstrap/external threads create an internal thread instance.
• Keep unrecognized classfile attributes as an Unknown variant instead of failing parsing; add an E2E fixture for currentThread() identity expectations.

Reviewed changes

Copilot reviewed 8 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test_data/src/CurrentThread.java	Adds an E2E Java fixture validating currentThread() reference identity across calls and threads.
test_data/CurrentThread.txt	Adds expected output for the CurrentThread E2E fixture.
jvm/src/thread.rs	Extends JvmThread to store an owned java/lang/Thread instance.
jvm/src/jvm.rs	Makes attach_thread accept/create a per-thread Java Thread object and adds current_java_thread().
jvm/src/garbage_collector.rs	Treats each thread’s stored Java Thread object as a GC root.
java_runtime/tests/classes/java/lang/test_object.rs	Updates test thread attach calls to the new async attach_thread(None) API.
java_runtime/src/classes/java/lang/thread.rs	Ensures Thread.start() attaches the spawned thread with the correct Java Thread receiver; updates currentThread() implementation.
classfile/src/attribute.rs	Preserves unknown attributes as AttributeInfo::Unknown instead of rejecting the classfile.

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