docs(dev/guide): align guides with enforced perfectionist rules

[KSXGitHub]

When a perfectionist rule starts enforcing a convention automatically, the prose that used to describe that convention by hand becomes a duplicate of the rule. The earlier lint!: upgrade perfectionist PR handled this for derive ordering by dropping the redundant quick-reference bullet and reframing the relevant CONTRIBUTING.md section as automatically enforced. This PR applies the same treatment to conventions that recent upgrades left duplicated, and corrects rule references that did not match the installed perfectionist version.

Changes

• Single-letter let bindings. The upgrade in lint: upgrade perfectionist #433 tightened single_letter_let_binding, which no longer permits single-letter bindings for interchangeable test fixtures (hence lint: upgrade perfectionist #433 renamed fixtures such as a/b/c). Remove the now-contradicted "test fixtures with identical roles" exception from CONTRIBUTING.md and the AI instruction templates, and require descriptive names for let bindings without the test-code exception.
• Import granularity. The switch to style = "module" in style: switch import granularity from "crate" to "module" #435 means perfectionist::import_granularity now enforces import merging automatically. Remove the redundant import bullet from the AI instruction quick reference, and reframe the CONTRIBUTING.md import section to lead with automatic enforcement, keeping only the hand-applied convention for platform-gated import blocks.
• Correct inaccurate rule references. Verified the single-letter guidance against perfectionist 0.0.0-rc.17 (the version pinned in dylint.toml) and fixed three claims that did not match the rules:
  • Removed the fabricated short_impl_max_lines knob; single_letter_generic has no configuration.
  • Renamed allowed_idents to extra_allowed_idents, the actual field name for the variable and closure rules.
  • Clarified that i/j/k are exempt as function and closure parameters but not as let bindings, where only n is allowed.

The AI instruction files (CLAUDE.md, AGENTS.md, .github/copilot-instructions.md) are regenerated from the templates via ./run.sh pdu-ai-instructions --generate ..

This branch has master merged into it, so it already incorporates #435.

https://claude.ai/code/session_01CMrfXtuzxpSbchSJNuwboK

[github-actions]

Performance Regression Reports

commit: 2a7a626

--quantity=apparent-size --max-depth=1 --min-ratio=0.01

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
pdu	100.9 ± 17.6	84.9	154.3	1.18 ± 0.21
pdu-0.20.0	85.6 ± 0.6	84.9	87.7	1.00

Logs

Benchmark 1: pdu
  Time (mean ± σ):     100.9 ms ±  17.6 ms    [User: 55.1 ms, System: 282.7 ms]
  Range (min … max):    84.9 ms … 154.3 ms    29 runs
 
Benchmark 2: pdu-0.20.0
  Time (mean ± σ):      85.6 ms ±   0.6 ms    [User: 55.2 ms, System: 279.1 ms]
  Range (min … max):    84.9 ms …  87.7 ms    34 runs
 
Summary
  pdu-0.20.0 ran
    1.18 ± 0.21 times faster than pdu

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