perf: cache InstrLocation across consecutive positions

[P403n1x87]

In ConcreteBytecode.from_code, InstrLocation._from_tuple is called once per instruction to construct a location object from the (lineno, end_lineno, col_offset, end_col_offset) tuple yielded by co_positions(). This was 5.31% of total CPU time in profiling.

Python bytecodes for a single source expression (e.g. a + b, a for loop header, a CACHE entry) all map to the same source position. Because of this structural property, position tuples repeat frequently and tend to be contiguous — consecutive instructions usually share the same position. In the dis module corpus, 78.4% of the 860 position tuples are repeated.

A single-entry cache (remember the last pos/loc pair) turns most iterations into a cheap tuple equality check, avoiding object.new + 4x object.setattr for the common case. A full dict cache was also tried but was slower: the per-iteration dict hash + lookup costs more than tuple equality even though it has a higher hit rate.

Performance analysis (own time):

Hotspot	Before	After
InstrLocation._from_tuple own	5.31%	1.55%

Code object analysis (dis module)

Metric	Value
Total position tuples	860
Unique positions	318 (37%)
Repeated positions	542 (63%)

Throughput (Bytecode.from_code().to_code() on dis module, 30 runs each) with Mann-Whitney U Test

Approach	p95	vs baseline
Baseline (main)	180 r/s	—
Dict cache	190 r/s	+5.6% (✓ significant, p≈0)
Single-entry cache	192 r/s	+6.7% (✓ significant, p≈0)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.45%. Comparing base (41c679b) to head (c2a610c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #202      +/-   ##
==========================================
+ Coverage   95.43%   95.45%   +0.01%     
==========================================
  Files           7        7              
  Lines        2126     2132       +6     
  Branches      458      459       +1     
==========================================
+ Hits         2029     2035       +6     
  Misses         54       54              
  Partials       43       43              

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