feat(core): add baseline/rhythm rule

[aram-devdocs]

Summary

• Adds the baseline/rhythm rule that flags text elements whose typographic baselines miss the configured vertical-rhythm grid (rhythm.base_line_px).
• Computes approximate baseline position from font-size, line-height, and bounding rect; fires when distance to nearest grid line exceeds rhythm.tolerance_px.
• Includes golden snapshot test, determinism test, rule docs page, SUMMARY.md and CHANGELOG entries.

Closes #73

Validations

• Rule module + registration in register_builtin (alphabetical order)
• Golden snapshot test (golden_baseline_rhythm_golden) with on-grid, off-grid, and non-text nodes
• Determinism test (baseline_rhythm_run_is_deterministic) — 3-run byte-identical check
• Rule docs at docs/src/rules/baseline-rhythm.md with all required sections
• docs/src/SUMMARY.md updated with alphabetically placed entry
• CHANGELOG [Unreleased] entry added
• git diff --check — no whitespace issues
• No unwrap/expect/todo!/dbg!/println! in library code
• Pure function of (snapshot, config) — no I/O, no wall-clock, no HashMap
• Uses IndexMap for metadata, parse_px from rules::util

Test plan

• cargo nextest run -p plumb-core --test golden_baseline_rhythm passes both golden and determinism tests
• just validate passes (fmt + clippy + full test suite)
• CI green

🤖 Generated with Claude Code

[github-actions]

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PR #207 Review — feat(core): add baseline/rhythm rule

File buckets touched: plumb-core (rule, snapshot, tests), plumb-cdp (text-box extraction), plumb-cli (selector rewrite), plumb-mcp (explain), plumb-format (SARIF snapshot), docs, ci/schema.

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1. Determinism

• IndexMap used for metadata ✓
• No HashMap/HashSet in observable output ✓
• text_boxes sorted (dom_order, start) before return in extract_text_boxes ✓
• Engine sort key (rule_id, viewport, selector, dom_order) is stable because Rust's sort is stable and insertion order within identical keys follows sorted text_boxes ✓
• No wall-clock, no env reads in plumb-core ✓

2. Blocker — multiple violations per node per viewport

crates/plumb-core/src/rules/baseline/rhythm.rs:342-393

The loop over y_origins emits one violation per off-grid text-box line. When a 4-line paragraph has 2 off-grid lines, the rule fires twice with identical (rule_id, viewport, selector, dom_order). This directly violates the documented invariant:

.agents/rules/rule-engine-patterns.md:70: "Don't emit more than one violation per offending node per viewport. Duplicate detection is caller-side; upstream fan-out burns token budget."

The existing tests don't catch this because baseline_rhythm_multiline_text_boxes is constructed with exactly one on-grid and one off-grid line — the double-off-grid case is untested.

Required fix: aggregate all off-grid lines into a single violation per node. The off-grid line details (baseline_y, distance, nearest grid) can go in metadata (e.g., "offgrid_lines": [...]) or the message can reference the worst/first offender.

3. Silent precondition in text_box_index

crates/plumb-core/src/snapshot.rs:505-518

text_box_index requires text_boxes to be sorted by (dom_order, start). The field is pub and serde-deserialized, so any caller loading a JSON snapshot with unsorted text boxes gets silently wrong text_boxes_for results — wrong index slices, potentially no violations where violations should fire. CDP output is always sorted, but test-fixture snapshots built from JSON could easily violate the invariant.

Required fix: either sort defensively in SnapshotCtx::new, or validate + return an error, or at minimum add a debug_assert that the slice is sorted so failures surface in test runs.

4. Missing double-off-grid test

Follows from the blocker above: the baseline_rhythm_multiline_text_boxes test cannot demonstrate the invariant violation because it only puts one line off-grid. The test suite needs a case where two text-box lines are off-grid for the same node, and asserts violations.len() == 1 after the fix lands.

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Non-blocking observations

• Cargo.lock shows subtle re-entering the plumb-mcp section, but it's already in crates/plumb-mcp/Cargo.toml:31 and lib.rs:71 for constant-time token comparison. This is a pre-existing dep; the lock-file churn is just regeneration. No concern.
• #[allow(clippy::cast_possible_truncation, clippy::cast_sign_loss)] in plumb-cdp/src/lib.rs:118-121 is narrowly scoped per-line and inside the unsafe-permitted crate with .max(0) guards. Acceptable.
• TEXT_TAGS.contains(&node.tag.as_str()) is O(31) linear scan in the hot loop. Fine under the "no new deps for a single rule" policy, but could become const PHFS if this ever shows in profiles.
• Violation rect is the element bounding box, not the off-grid text-box line rect. Intentional or not, it means the fix hint can point to the right grid line but the rect highlights the whole element. Worth noting in follow-up.

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Punch list:

#	File:line	Issue	Class
1	crates/plumb-core/src/rules/baseline/rhythm.rs:341-393	Loop emits N violations per node; one-per-node invariant violated	BLOCKER
2	crates/plumb-core/src/snapshot.rs:505-518	text_box_index silently wrong if text_boxes unsorted at deserialize time	BLOCKER
3	crates/plumb-core/tests/golden_baseline_rhythm.rs	Missing double-off-grid multiline test	Warning

Verdict: REQUEST_CHANGES