enhance(#3783): audit-uat summary segments current-milestone vs archived debt (#4336)

* test(#3783): add failing coverage for audit-uat summary segmentation

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* fix(#3783): segment audit-uat summary into current_milestone and archived buckets

Additive: current_milestone/archived are new; total_items, total_files, parse_gap_files, by_phase, and by_category are unchanged.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* docs(#3783): add changeset fragment for audit-uat summary segmentation

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* chore(#3783): allowlist the new audit-uat-summary-segmentation test file

lint-test-file-count.cjs baselines the "audit" module (keyed off bin/lib/audit.cjs)
at 6 pre-existing files; this adds the new dedicated suite as a 7th, matching the
module's existing one-file-per-feature-slice precedent.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* test(#3783): fix phase/file number mismatch in the mixed-milestone fixture

The active phase fixture used dir "02-current" with file "01-UAT.md" — a
cross-phase stray per phase-id.cts's isPhaseArtifact/scopeToPhase (#3511),
so the file was silently excluded from the scan and current_milestone read
{files:0, items:0} instead of {files:1, items:1}. Confirmed by direct CLI
run against a hand-built fixture before recommitting. Renamed the file to
02-UAT.md to match its directory's phase number, matching every other
fixture in this suite.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* docs(#3783): backfill changeset PR number to 4336

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

---------

Co-authored-by: sim <sim@local>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

scripts/lint-test-file-count.allowlist.json

@@ -261,9 +261,10 @@
         "audit-milestone-filename-guard.test.cjs",
         "audit-open-remainder-count.test.cjs",
         "audit-uat-acknowledged.test.cjs",
+        "audit-uat-summary-segmentation.test.cjs",
         "audit-workstream-layouts.test.cjs"
       ],
-      "issue": "#3804/#3805 \u2014 audit-layout and acknowledged-marker suites drive the real audit-uat CLI against on-disk fixtures | #3817 \u2014 regression suite proving countReal counts the truncation remainder marker instead of treating it as one phantom item"
+      "issue": "#3804/#3805 \u2014 audit-layout and acknowledged-marker suites drive the real audit-uat CLI against on-disk fixtures | #3817 \u2014 regression suite proving countReal counts the truncation remainder marker instead of treating it as one phantom item | #3783 \u2014 dedicated suite for the summary.current_milestone/archived segmentation, matching this module's one-file-per-feature-slice precedent (e.g. audit-uat-acknowledged.test.cjs) rather than growing the unrelated 6900+ line tests/uat.test.cjs (a different module, keyed off uat.cjs)"
     },
     "shell-command-projection": {
       "files": [