feat(pipeline): plain-English per-step result summaries

Replaces the raw-JSON summary column in the Results table with the mockup's
plain-English phrasing: "312 duplicates removed across 147 groups
(18,442 → 18,130 rows)", "1,204 cells cleaned in name & city", etc.
(correct singular/plural via a small _n helper).

Adds step_phrase() and step_status() to pipeline_modules.py. step_status
derives the status pill (✓ ok / ⚠ ok · N skipped / ✗ error / ⏭ skipped) and,
for warn/error steps (e.g. format_standardize unparseable cells, column_map
coercion failures / missing required targets), an inline detail callout
rendered directly below the results table — surfacing non-fatal issues in
context without a dedicated always-empty column.

Extends tests/gui/test_pipeline_builder.py with phrasing + status assertions.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/gui/test_pipeline_builder.py

@@ -89,3 +89,32 @@ def test_run_produces_results_with_friendly_names():
     res = at.session_state["pipeline_result"]
     assert res.initial_rows == 3 and res.final_rows == 2  # the two Jane rows merge
     assert all(sr.error is None for sr in res.step_results)
+
+
+def test_step_phrase_is_plain_english_not_json():
+    from src.gui.components.pipeline_modules import step_phrase, step_status
+
+    # dedup phrasing mirrors the design mockup wording exactly.
+    phrase = step_phrase("dedup", {
+        "input_rows": 18442, "output_rows": 18130,
+        "duplicates_removed": 312, "groups": 147,
+    })
+    assert phrase == "312 duplicates removed across 147 groups (18,442 → 18,130 rows)"
+
+    # text_clean lists affected columns in prose, with thousands separators.
+    assert step_phrase("text_clean", {
+        "cells_changed": 1204, "columns_processed": ["name", "city"],
+    }) == "1,204 cells cleaned in name & city"
+
+    # singular nouns pluralize correctly
+    assert step_phrase("missing", {"rows_dropped": 1, "columns_dropped": ["x"]}) == \
+        "1 row dropped, 1 column dropped"
+
+    # unparseable cells downgrade the pill to warn with an inline detail
+    label, level, detail = step_status(
+        "format_standardize", {"cells_changed": 100, "cells_unparseable": 141},
+    )
+    assert level == "warn" and "141 skipped" in label and detail
+
+    # a clean step is "ok" with no detail
+    assert step_status("text_clean", {"cells_changed": 5})[1] == "ok"