perf: cache has_jump flag and pass buffer in _pack_location

[P403n1x87]

Two independent micro-optimisations on the roundtrip hot path, benchmarked together:

1. Cache _is_jump on BaseInstr at construction time

has_jump() is called on every instruction in ControlFlowGraph.from_bytecode, BasicBlock.append, and _StackSizeComputer.run — the profiler showed it at ~2.9% own time, spending that time on opcode in HAS_JUMP (a set lookup) on every call.

Added _is_jump: bool to BaseInstr.slots and compute it once in _set() (the canonical setter used by init and the public set() method). All fast-path constructors that bypass _set() — copy(), _from_trusted() on both BaseInstr and ConcreteInstr, and _from_opcode() on ConcreteInstr — now copy or compute the flag directly. has_jump() becomes a single slot read.

2. Eliminate per-call bytearray allocation in _pack_location

_assemble_locations previously collected one bytearray per location group via _push_locations -> _pack_location -> bytearray(), then joined them with b"".join(locations) at the end. Each location entry is only 2-6 bytes, so the list of small bytearrays and the final join were measurable overhead (_pack_location at ~3.9% own in the profiler).

Changed the signature of _pack_location and _push_locations to accept a shared bytearray buf and extend into it in place. _assemble_locations creates one bytearray() up-front and converts to bytes at the end -- zero intermediate allocations.

Benchmark (perf.py, Bytecode.from_code(dis).to_code(), 30 runs, p95 r/s):

	p95 (r/s)	95% CI
Baseline	188	[187, 188]
This PR	196	[195, 196]

Delta: +8 r/s (+4.3%), Mann-Whitney p~0 (significant, threshold: p<0.01 and |delta|>=2%)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.45%. Comparing base (40b6bd4) to head (df38049).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #203   +/-   ##
=======================================
  Coverage   95.45%   95.45%           
=======================================
  Files           7        7           
  Lines        2132     2135    +3     
  Branches      459      459           
=======================================
+ Hits         2035     2038    +3     
  Misses         54       54           
  Partials       43       43           

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