Fix decode_instructions panics on malformed inputs

[Copilot]

Fuzz targets were crashing (exit code 77) because decode_instructions had four panic sites reachable with malformed transaction data.

Root causes

• account_keys[ci.program_id_index as usize] — unbounded index into account keys array
• account_keys[i as usize] — same for per-instruction account indices
• account_keys[0] — panics on empty account keys
• panic!("No visualizer available...") — explicit panic when no visualizer matches

Changes

• src/core/instructions.rs: Replace map() + bare indexing with filter_map() that skips instructions whose program_id_index or account indices are out of bounds — mirrors the already-safe decode_v0_instructions path. Use .first() with a default-pubkey fallback for the sender key. Replace the explicit panic! with ok_or_else(|| VisualSignError::ParseError(...)).

// Before: panics on malformed input
let instructions: Vec<Instruction> = message.instructions.iter().map(|ci| Instruction {
    program_id: account_keys[ci.program_id_index as usize],  // 💥
    ...
}).collect();

// After: skips invalid instructions gracefully
let instructions: Vec<Instruction> = message.instructions.iter().filter_map(|ci| {
    let idx = ci.program_id_index as usize;
    if idx >= account_keys.len() { return None; }
    ...
}).collect();

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[shahan-khatchadourian-anchorage]

Closing since this was just a small experiment.