ci: make rustfmt policy stable-compatible

[KooshaPari]

Summary

• remove nightly-only rustfmt options from the shared formatter policy
• normalize one existing rustdoc whitespace violation

Recovery provenance

Reconstructed narrowly from historical branches fix/stable-rustfmt-policy-adfa0978 and fix/stable-rustfmt-policy-c2cd6fd on current origin/main; original refs remain retained.

Verification

• cargo fmt --all -- --check
• git diff --check

Co-Authored-By: ForgeCode noreply@forgecode.dev

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