Fix 7 opcode interpreter bugs to match BSV node v1.2.0 exactly

[sirdeggen]

Identified and fixed the following discrepancies between the py-sdk
interpreter (bsv/script/spend.py) and the reference BSV node interpreter
(bitcoin-sv/bitcoin-sv src/script/interpreter.cpp @ v1.2.0):

1. OP_LSHIFT / OP_RSHIFT

   • Was doing byte-level shifts; now performs bit-level shifts matching
     the node's LShift()/RShift() helpers using mask tables.
   • Was not popping n from the stack (left a stale value).
   • Added proper handling: n >= data_bits → zero-fill entire result.

2. OP_INVERT

   • ~b in Python yields -(b+1) for unsigned bytes, causing TypeError or
     wrong results. Fixed to ~b & 0xFF.

3. OP_DIV

   • Python // is floor division (toward -inf); C / truncates toward zero.
     Fixed with sign * (abs(a) // abs(b)).

4. OP_MOD

   • Python % sign follows divisor; C % sign follows dividend.
     Fixed with a - b * trunc_div(a, b).

5. OP_NUM2BIN

   • msb was initialised as b'\x00' (bytes) instead of 0 (int), causing
     TypeError when OR-ing into the bytearray for a zero input.

6. OP_CHECKMULTISIG

   • Unconditional sigs_count -= 1 should be keys_count -= 1, matching
     the node's nKeysCount-- after each key iteration.

7. is_chunk_minimal

   • OpCode.OP_1 + n.to_bytes(…) was bytes concatenation (2 bytes),
     so values 1–16 pushed via direct-push were never flagged as
     non-minimal. Fixed with integer arithmetic.

Also adds tests/test_opcodes.py with 49 tests covering all fixed opcodes
including bit-shift helpers, arithmetic edge cases with negative numbers,
and NUM2BIN/BIN2NUM round-trips.

https://claude.ai/code/session_01AnvbrLbXwdionxzd9askb3

[sonarqubecloud]

Quality Gate passed

Issues
10 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud