feat(core,view): ③ 공통 설정 — fck and fy as inputs (spec §6.2)

- Material {fck, fyH, fyS}, defaults 27 / 400 / 500 as assumed values (I02).
  The rebar calculations take the material: the C7/C8 splice, the P2 development
  length, the P3 lap, and the B2 split use it instead of fixed numbers.
- ③ screen: 공통 설정 inputs with a note that the defaults are assumed (I02).
- Project file stores the material as an optional field; older files open with
  the defaults.
- Tests: fck 27 → 35 shortens the laps by √(27/35); the C7 piece follows fck;
  a non-default fy drops I02; the file round trip.

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>

index.html

@@ -138,6 +138,13 @@
         <aside class="panel piers" aria-labelledby="rebar-member-title">
           <p id="rebar-member-title" class="lbl">부재</p>
           <ul id="rebar-tabs" class="members"></ul>
+          <section class="common" aria-labelledby="common-title">
+            <p id="common-title" class="lbl">공통 설정</p>
+            <label class="mat">fck (MPa) <input id="mat-fck" type="number" step="any" /></label>
+            <label class="mat">fy H · SD400 (MPa) <input id="mat-fyh" type="number" step="any" /></label>
+            <label class="mat">fy S · SD500 (MPa) <input id="mat-fys" type="number" step="any" /></label>
+            <p id="mat-note" class="muted small"></p>
+          </section>
           <p class="pnote">형상은 ② 형상계획 기준입니다. 형상이 바뀌면 "형상 바뀜"으로 표시합니다.</p>
         </aside>
         <section class="panel viewer" aria-labelledby="rebar-title">