feat(view): round bar bends at the centre-line radius; bend factors as inputs on ③

Bars drawn through several points are one tube each, with every bend rounded by R·tan(φ/2) at the centre-line radius
(inner factor × d + d/2). The inside-radius factors (D16 and below, D19–D25, D29–D35, D38 and above) are inputs in 공통
설정, with KDS-based defaults 2 / 3 / 4 / 5. The size-class limits sit in one place (SIZE_CLASS), which also fixes
D25 (25.4) being read as the D29–D35 class in hookLength. The helper bendDeduction gives the centre-line deduction for
the length option, which is not yet applied to the 철근표 lengths.

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

index.html

@@ -181,6 +181,11 @@
             <label class="mat">D 규격 강도 <select id="mat-grade-d"><option value="300" selected>SD300</option><option value="400">SD400</option><option value="500">SD500</option></select></label>
             <label class="mat">H 규격 강도 <select id="mat-grade-h"><option value="300">SD300</option><option value="400" selected>SD400</option><option value="500">SD500</option></select></label>
             <label class="mat">S 규격 강도 <select id="mat-grade-s"><option value="300">SD300</option><option value="400">SD400</option><option value="500" selected>SD500</option></select></label>
+            <p class="lbl">굽힘 내면반경 (× 철근 지름)</p>
+            <label class="mat">D16 이하 <input id="bend-small16" type="number" step="0.5" min="0.5" /></label>
+            <label class="mat">D19–D25 <input id="bend-main25" type="number" step="0.5" min="0.5" /></label>
+            <label class="mat">D29–D35 <input id="bend-main35" type="number" step="0.5" min="0.5" /></label>
+            <label class="mat">D38 이상 <input id="bend-main51" type="number" step="0.5" min="0.5" /></label>
             <p id="mat-note" class="muted small"></p>
           </section>
         </aside>