fix(docs): drop the unused locals ruff has been failing main on

[kelvin1295]

main's CI has been red on lint for at least the last five pushes — every one a docs commit from the orchestration-patterns work, none of them touching the code the failure is in. This turns it green.

The six

All in docs/model_orchestration_patterns/build_diagrams.py:

where	rule	what
lrange	F841 ×2	base, bh unpacked, never read
relaxation block	F401	import math — no math. anywhere in the file
canvas clamp	F841	bw unpacked, never read
_elabel	F841 ×2	bw, bh unpacked, never read

bh at the canvas clamp stays — it is read two lines below, at the box[1] = min(max(...)) clamp. Only bw goes there.

Nothing computed is lost: each site reads a list by index, so dropping the name drops no work.

Test plan

• ruff check . → All checks passed
• Behaviour-preserving, proven by regenerating the diagrams: python3 build_diagrams.py rewrites every committed SVG byte-identically after the change — git status shows only the .py itself
• Positive control run first: the unmodified script also reproduces the committed SVGs exactly, so the check is capable of seeing a difference rather than passing vacuously

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