fix(alerts): quieter Inbox and a tidier Rules table - #2451

🟢 Confidence 10/10 · safe to merge. Only real behavior change is NEAR_HEADROOM 20%→10% (both pages) and the __total__ scope fix, each pinned by new tests.
_{no findings · tests covered · risk medium}

Polish for the alerting Inbox and Rules table: one headroomTone helper shared by both pages (with a breach tone), a fixed-width rules search, hidden lone "No tag" header, ungrouped incidents losing their "on ..." suffix, and a quieter Inbox toolbar. Safe to merge.

• headroomTone + HEADROOM_FILL/HEADROOM_TEXT added, shared by the Rules meter and Inbox bars
• NEAR_HEADROOM drops from 20% to 10% in overview-derive
• isLoneUntagged hides the sole "No tag" group header in the rules table
• inboxFilters returns nothing for an org with no anomaly incident in the window

_{761f02c · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}