feat(core): rule edge/near-alignment

[aram-devdocs]

Summary

Cluster edges within alignment.tolerance_px (default 3) and
flag elements off by 0 < delta ≤ tolerance — the "almost but
not quite aligned" case.

Crate / area

plumb-core — rule engine, types, determinism (no internal deps)

Phase / batch

Part of #20 → Batch 2B (Remaining MVP rules (5 parallel implementations)).

Effort: M

PRD references

• PRD §11.2
• PRD §11.3

Acceptance criteria

• Edge clustering algorithm documented + tested.
• Rule emits violation for near-miss alignments, ignores pixel-perfect and unrelated edges.
• Golden test + docs + register_builtin.

Dependencies

• None (first in batch; batch-level deps gate this).

Review gates

• 02-spec-reviewer
• 03-code-quality-reviewer
• 05-architecture-validator
• 04-test-runner

How to deliver

Dispatch /gh-issue <this-issue-number>. See the root AGENTS.md and the scoped plumb-core/AGENTS.md for the invariants that apply.

Suggested delivery: gh-issue