opt(interpreter): use feed_direct() in compute_block_gas_costs()

[liuyiyi185]

Summary

Replace the ast_cost_from_raw() + sim.feed(&fc) path in
compute_block_gas_costs() with a direct sim.feed_direct() fast path
for ~90% of instructions.

Changes

• Before: Every instruction went through ast_cost_from_raw() (which
  constructs a FastCost struct with bitmask arithmetic, trailing_zeros
  extraction, etc.) and then sim.feed(&fc).
• After: ~90% of instructions (loads, stores, ALU ops, immediates,
  context-free terminators) call sim.feed_direct(cycles, slots, src1, src2, dst)
  directly with raw register nibbles - no struct allocation, no bitmask ops.
  Only instructions that genuinely require context-dependent cost computation
  (ecalli 10, load_imm_jump 80, conditional branches 181..=255, and
  unrecognised opcodes) fall back to the full ast_cost_from_raw() + eed().

Performance Impact

• Eliminates FastCost struct construction for the common-case path
• Reduces memory pressure in compute_block_gas_costs() hot loop
• No allocations changed in the runtime interpreter - this only affects the
  one-time pre-decode path, but also hits Interpreter::predecode() used
  by CODE cap construction

Testing

• All existing interpreter tests pass (cargo test -p javm)
• Gas cost calculations remain identical: eed_direct() is already verified
  to produce the same result as eed(&FastCost{...}) by the
  eed_matches_feed_direct proptest in gas_sim.rs

Related

Closes #812 - optimize javm interpreter performance (#400)

[github-actions]

Genesis Review

Comparison targets:

• fcc0b6d0 (fcc0b6d)
• dc0f542f (dc0f542)
• a1a8ae03 (a1a8ae0)
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• 05b0f900 (05b0f90)
• 37939ba1 (37939ba)
• 94a39123 (94a3912)

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