perf: bypass ConcreteInstr validation in from_code via _from_opcode

[P403n1x87]

Add ConcreteInstr._from_opcode, a fast-path constructor for the from_code loop that skips all validation and sets the six slots directly via object.new + attribute assignment.

The checks are safe to skip because every instruction in this path comes from CPython's own co_code bytes:

• name/opcode: _opcode.opname[byte] is always a valid opcode
• arg: a raw byte (0-255), always satisfies _check_arg_int, or UNSET for no-argument opcodes (already gated by opcode_has_argument)
• _size: always 2 — EXTENDED_ARG accumulation happens after this loop in _remove_extended_args, not during construction
• _extended_args: always None in this path

CPU profiling data (where the impact is more visible):

Hotspot	Before	After
ConcreteInstr.__init__ own	3.11%	eliminated
ConcreteInstr._check_arg own	1.82%	eliminated
ConcreteBytecode.from_code own	6.21%	4.55%

Throughput (usual Bytecode.from_code().to_code() round-trip, 1 second timed window, 5 runs):

	r/s range
Before	~143
After	~148

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.19%. Comparing base (25bf1bc) to head (fd74bbf).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #197      +/-   ##
==========================================
+ Coverage   95.17%   95.19%   +0.02%     
==========================================
  Files           7        7              
  Lines        2051     2061      +10     
  Branches      446      446              
==========================================
+ Hits         1952     1962      +10     
  Misses         55       55              
  Partials       44       44              

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