fix: address 3 critical security issues (fixes #15, #16, #17)

[SuperInstance]

Summary

#15: Zero Bytecode Verification

• Added verify_bytecode() static method to Interpreter class
• Rejects empty bytecode with clear ValueError
• Rejects bytecode exceeding 1 MB (MAX_BYTECODE_SIZE)
• Verification runs at the top of execute() before the fetch-decode loop

#16: CAP Opcodes Unenforced

• Added capabilities: set[int] attribute to the VM interpreter
• CAP_GRANT(reg): adds capability ID to the set, writes 0 (success) to R0
• CAP_REVOKE(reg): removes capability ID from the set, writes 0 (success) to R0
• CAP_REQUIRE(reg): checks if capability is in the set; raises VMA2AError if missing
• CAP_REQUEST(reg): forwarded to external A2A handler (no auto-grant)
• Capabilities cleared on reset()

#17: NaN Trust Poison

• Added NaN detection via math.isnan() in record_interaction() for capability_match, behavior_signature, and latency_ms
• Added NaN detection in check_trust() for threshold
• All NaN values raise ValueError with descriptive message
• capability_match clamped to [0.0, 1.0] on input
• threshold clamped to [0.0, 1.0] on input

Testing

• 14 new regression tests in tests/test_security.py
• Full suite: 2378 passed, 0 failed, 50 warnings
• No existing tests broken

Files Changed

• src/flux/vm/interpreter.py — bytecode verification + CAP enforcement
• src/flux/a2a/trust.py — NaN protection + clamping
• tests/test_security.py — regression tests for all 3 fixes

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[beta-devin-ai-integration]

Devin Review found 1 potential issue.

View 4 additional findings in Devin Review.

[beta-devin-ai-integration]

🟡 Docstring omits latency_ms from Raises section despite NaN check in code

The record_interaction docstring's Raises section states If *capability_match* or *behavior_signature* is NaN, but the code at src/flux/a2a/trust.py:165-168 also raises ValueError when latency_ms is NaN. A caller relying on the docstring to understand which parameters are validated would not know that NaN latency_ms also raises. The test test_nan_latency_raises at tests/test_security.py:199-207 confirms this behavior exists, making the docstring inaccurate.

Suggested change

	Raises
	------
	ValueError
	If *capability_match* or *behavior_signature* is NaN.
	Raises
	------
	ValueError
	If *capability_match*, *behavior_signature*, or *latency_ms* is NaN.

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