docs(papers): Update MIR13 to MIR14 and create SSA construction paper

Major changes:
- Update all MIR13 references to MIR14 throughout paper-a-mir13-ir-design/
- Add evolution history: 27 → 13 → 14 instructions (UnaryOp restoration)
- Create new paper-d-ssa-construction/ for SSA implementation struggles
- Add PAPER_INDEX.md consolidating ChatGPT5's 3-paper analysis

MIR14 updates:
- README.md: Add instruction evolution timeline
- abstract.md: Emphasize practical balance over pure minimalism
- main-paper*.md: Update titles and core concepts
- MIR13_CORE13_SPEC.md: Add UnaryOp to instruction list
- chapters/01-introduction.md: Reframe as "14-Instruction Balance"
- RENAME_NOTE.md: Document folder naming consideration

SSA paper structure:
- README.md: Paper overview and positioning
- current-struggles.md: Raw implementation challenges
- technical-details.md: BuilderCursor, Sealed SSA, type normalization
- abstract.md: English/Japanese abstracts

LoopForm experiments continue in parallel (minor adjustments to detection).

🤖 Generated with [Claude Code](https://claude.ai/code)

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

apps/tests/loop_min_while.nyash

@@ -0,0 +1,11 @@
+static box Main {
+  main() {
+    local i = 0
+    loop(i < 3) {
+      print(i)
+      i = i + 1
+    }
+    return 0
+  }
+}
+