feat(adapter): Phase 6 Slice B — mBrian-backed read path live

Per t-projax-6-sliceB-readpath. mBrian migration (m/mBrian#73) is live
on msupabase with 65 nodes + 78 child_of + 81 projax-* edges. This
commit makes the projax read path source from there behind an env
switch.

CLIENT ARCH: direct pgxpool against mbrian.* schema (same
SUPABASE_DATABASE_URL the projax binary already uses for projax.*) —
matches flexsiebels/head's cross-coupling pattern. No MCP token
plumbing.

CONTRACT (all three honoured)
- External links are SELF-EDGES (source=target=item, rel='projax-*',
  payload in edges.metadata). linkFromEdge reads the node's outbound
  projax-* edges; ref_id derived per ref_type from metadata (caldav
  url, gitea owner/repo, mai-project mai_project_id).
- Slugs finalised: 'work'/'dania' resolve to mBrian's canonical nodes;
  projax-side squatters (renamed-aside, not deleted) are documented in
  the parity test as legacy-only and skipped from field comparison.
- created_at/updated_at NOT preserved — ItemsCreatedInRange orders off
  metadata.projax.start_time when present, fall back to mBrian
  created_at. Aggregator surfaces (timeline / dashboard) read off
  caldav DTSTART + gitea updated_at, so they're unaffected.

NEW FILES
- store/mbrian.go: MBrianReader concrete impl. Bulk-loads projax-
  managed nodes + child_of edges in one pair of queries per call,
  builds a graphContext in memory, derives Paths via ancestor walk
  (depth-capped at 64 like projax's trigger). Implements every
  ItemReader method.
- store/mbrian_parity_test.go: 5 parity tests against the live db —
  ListAll field equality (skipping the renamed squatter slugs),
  spot-check resolves, caldav-list link round-trip, gitea-repo link
  round-trip, AllTags union, NotFound consistency. All 5 GREEN.
- cmd/projax-remap-views/main.go: one-shot tool to rewrite
  projax.views.filter_json.project_id from old projax uuids to new
  mBrian uuids using the audit map mBrian dropped (head will relay
  the path). Dry-run default; --apply commits. Idempotent.
- docs/plans/slice-b-views-projectid-gap.md: surfaces the gap + the
  remediation path. Must run remap BEFORE slice E drops projax.items.

CHANGES
- store/adapter.go: kept the ItemReader interface + *Store assertion;
  removed the prep stub (replaced by mbrian.go).
- web/server.go: Server.Items store.ItemReader field. web.New defaults
  Items to the concrete *Store (legacy path). main.go overrides to
  MBrianReader when PROJAX_BACKEND=mbrian.
- All read-path call sites in web/ swapped from s.Store.<readMethod>(
  to s.Items.<readMethod>( for the 15 ItemReader methods. MCP tools
  unchanged (separate scope; can pivot in a follow-up). Writes still
  flow through s.Store.
- cmd/projax/main.go: PROJAX_BACKEND env switch with "store" (default)
  and "mbrian" values. Logs the choice at startup. Unknown value
  refuses to start.

SMOKE
- go build ./... green; go vet green.
- go test ./store/ -count=1 — all parity tests pass against live data.
- Local server boot with PROJAX_BACKEND=mbrian — backs binding logs
  "backend=mbrian (read path via store.MBrianReader)" and serves
  /views/tree (auth wall protects deeper smoke; parity tests cover
  that surface).

PRE-EXISTING failure NOT addressed in this commit: 3 timeline_filter
tests in web/ already failed on main (legacy /timeline URL hits the
Phase 5j 301 redirect to /views/timeline). No diff vs main in those
test files; out of scope for slice B.

OUT OF SCOPE FOR SLICE B (deferred):
- MCP read tools migration to ItemReader (separate diff, low risk).
- Aggregator's LinkLister wired to ItemReader (currently consumes
  *Store directly through Server.Aggregator()).
- views.filter_json.project_id remap RUN — tool ships here, run waits
  on the head's relay of the audit-map path.
- Slice C write-path. Slice D mai-bridge worker. Slice E drop.

web/graph.go

@@ -41,7 +41,7 @@ type graphPayload struct {
 }
 
 func (s *Server) handleGraph(w http.ResponseWriter, r *http.Request) {
-	items, err := s.Store.ListAll(r.Context())
+	items, err := s.Items.ListAll(r.Context())
 	if err != nil {
 		s.fail(w, r, err)
 		return
@@ -51,7 +51,7 @@ func (s *Server) handleGraph(w http.ResponseWriter, r *http.Request) {
 		s.fail(w, r, err)
 		return
 	}
-	allTags, err := s.Store.AllTags(r.Context())
+	allTags, err := s.Items.AllTags(r.Context())
 	if err != nil {
 		s.fail(w, r, err)
 		return