MVP M4: SysML metamodel + validator + dependency graph + UI surfacing

apps/web/lib/sysml — pure engine (no UI deps)
- model.ts: canonical types ported from phase-0 (Block, Property,
  Association, Constraint, Requirement, SysMLModel)
- validate.ts: 12 rules grouped Structural (S1–S5) / Semantic (M1–M5) /
  Traceability (T1–T2). Pure function returning element-anchored issues.
  Cycle detection via DFS with canonical-key dedup. T3 deferred to Phase 1.5
  (depends on narrative chip references).
- depgraph.ts: directed graph over Blocks / Properties / Constraints /
  Requirements with `dependentsOf()` and `neighborhood()` for impact analysis
  (substrate for M7).
- fromFixture.ts: loose FixtureData → strict SysMLModel converter (interim
  until M5 unifies state).
- breaks.ts: 8 named model corruptions (S1/S2/S3/M2/M3/M5/T1/T2) for
  ?break=... demonstrations.

UI integration in EditorShell + LeftRail + DiagramCanvas + IssuesPanel
- Floating IssuesPanel (bottom-right) with severity-grouped counts and
  collapse. Click an issue to focus its anchor across the rail and diagram.
- LeftRail: severity dots on each block in the Model section and on each
  requirement entry. Tooltips show full issue text.
- DiagramCanvas: red/blue/dotted ring around offending blocks via
  issuesByElement → BlockNode.data.issueSeverity.
- ?break=S1-dangling,M2-cycle,M5-dup-property,... query param injects
  corruptions for verification of the "done when" criterion.
- Suspense boundary added at the editor route for useSearchParams.

Two pre-existing TipTap typing fixes (editor.storage cast through unknown).

apps/web/styles/base.css

@@ -923,3 +923,123 @@ button { font-family: inherit; }
 .tiptap .ProseMirror-selectednode .chip {
   box-shadow: 0 0 0 1.5px var(--accent);
 }
+
+/* ─── Issues panel (M4) ─── */
+.issues-panel {
+  position: fixed;
+  bottom: 32px;
+  right: 16px;
+  width: 380px;
+  max-height: 50vh;
+  background: var(--surface);
+  border: 1px solid var(--border-strong);
+  border-radius: 6px;
+  box-shadow: 0 8px 24px var(--shadow-strong);
+  display: flex;
+  flex-direction: column;
+  z-index: 50;
+  font-family: var(--font-body);
+  overflow: hidden;
+}
+.issues-panel-clean {
+  width: auto;
+  min-width: 200px;
+}
+.issues-panel-clean .issues-panel-head { color: var(--ok-strong); }
+.issues-panel-clean-glyph {
+  color: var(--ok);
+  font-size: 14px;
+  font-weight: 600;
+}
+.issues-panel-collapsed {
+  max-height: none;
+}
+.issues-panel-head {
+  display: flex; align-items: center; gap: 10px;
+  padding: 8px 12px;
+  background: transparent;
+  border: none;
+  border-bottom: 1px solid var(--border);
+  cursor: pointer;
+  font-family: inherit;
+  color: var(--fg);
+  width: 100%;
+  text-align: left;
+}
+.issues-panel-collapsed .issues-panel-head { border-bottom: none; }
+.issues-panel-head:hover { background: var(--surface-2); }
+.issues-panel-title {
+  font-family: var(--font-display);
+  font-weight: 500;
+  font-size: 13px;
+  flex: 1;
+}
+.issues-panel-counts { display: flex; gap: 8px; }
+.issues-count {
+  font-family: var(--font-mono);
+  font-size: 11px;
+  font-weight: 500;
+}
+.issues-count-error { color: var(--warn-strong); }
+.issues-count-warning { color: var(--accent-strong); }
+.issues-count-soft { color: var(--muted); }
+.issues-panel-caret { font-size: 9px; color: var(--muted); }
+
+.issues-panel-list {
+  list-style: none;
+  margin: 0;
+  padding: 4px;
+  overflow-y: auto;
+  flex: 1;
+}
+.issues-item {
+  display: grid;
+  grid-template-columns: 16px auto 1fr;
+  gap: 8px;
+  padding: 6px 10px;
+  border-radius: 4px;
+  cursor: pointer;
+  font-size: 12.5px;
+  line-height: 1.35;
+}
+.issues-item:hover { background: var(--surface-2); }
+.issues-item-error .issues-item-sev { color: var(--warn-strong); }
+.issues-item-warning .issues-item-sev { color: var(--accent); }
+.issues-item-soft .issues-item-sev { color: var(--muted); }
+.issues-item-sev { font-size: 11px; line-height: 1.6; text-align: center; }
+.issues-item-code {
+  font-family: var(--font-mono);
+  font-size: 10.5px;
+  color: var(--muted-strong);
+  background: var(--surface-2);
+  padding: 1px 5px;
+  border-radius: 3px;
+  align-self: start;
+  margin-top: 1px;
+}
+.issues-item-msg { color: var(--prose); }
+
+/* Issue dots in the rail */
+.rail-issue-dot {
+  width: 6px; height: 6px;
+  border-radius: 50%;
+  margin-left: auto;
+  flex-shrink: 0;
+  margin-right: 4px;
+}
+.rail-issue-dot-error { background: var(--warn-strong); box-shadow: 0 0 0 2px var(--warn-soft); }
+.rail-issue-dot-warning { background: var(--accent); box-shadow: 0 0 0 2px var(--accent-soft); }
+.rail-issue-dot-soft { background: var(--muted); }
+
+/* Diagram node issue ring */
+.sysml-node-issue-error {
+  border-color: var(--warn-strong) !important;
+  box-shadow: 0 0 0 2px var(--warn-soft), 0 2px 4px var(--shadow);
+}
+.sysml-node-issue-warning {
+  border-color: var(--accent) !important;
+  box-shadow: 0 0 0 2px var(--accent-soft), 0 2px 4px var(--shadow);
+}
+.sysml-node-issue-soft {
+  border-style: dotted !important;
+}