fix(mir/builder): use function-local ValueId throughout MIR builder

Phase 25.1b: Complete SSA fix - eliminate all global ValueId usage in function contexts.

Root cause: ~75 locations throughout MIR builder were using global value
generator (self.value_gen.next()) instead of function-local allocator
(f.next_value_id()), causing SSA verification failures and runtime
"use of undefined value" errors.

Solution:
- Added next_value_id() helper that automatically chooses correct allocator
- Fixed 19 files with ~75 occurrences of ValueId allocation
- All function-context allocations now use function-local IDs

Files modified:
- src/mir/builder/utils.rs: Added next_value_id() helper, fixed 8 locations
- src/mir/builder/builder_calls.rs: 17 fixes
- src/mir/builder/ops.rs: 8 fixes
- src/mir/builder/stmts.rs: 7 fixes
- src/mir/builder/emission/constant.rs: 6 fixes
- src/mir/builder/rewrite/*.rs: 10 fixes
- + 13 other files

Verification:
- cargo build --release: SUCCESS
- Simple tests with NYASH_VM_VERIFY_MIR=1: Zero undefined errors
- Multi-parameter static methods: All working

Known remaining: ValueId(22) in Stage-B (separate issue to investigate)

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

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

lang/src/compiler/pipeline_v2/map_helpers_box.hako

@@ -12,7 +12,7 @@ static box MapHelpersBox {
   get_str(m, key) {
     local v = me._raw_get(m, key)
     if v == null { return "" }
-    if BoxHelpers.is_map(v) == 1 {
+    if BoxHelpers.is_map(v) {
       local inner = BoxHelpers.map_get(v, "value")
       if inner != null { return "" + inner }
     }

lang/src/compiler/pipeline_v2/normalizer_box.hako

@@ -7,7 +7,7 @@ static box NormalizerBox {
   normalize_cmp(raw) {
     if raw == null { return null }
     // TEMP debug
-    if BoxHelpers.is_map(raw) == 1 && BoxHelpers.map_get(raw, "cmp") != null { print("DBG:N.cmp_in=1") } else { print("DBG:N.cmp_in=0") }
+    if BoxHelpers.is_map(raw) && BoxHelpers.map_get(raw, "cmp") != null { print("DBG:N.cmp_in=1") } else { print("DBG:N.cmp_in=0") }
     local out = new MapBox()
     // cmp
     local cmp = "" + BoxHelpers.map_get(raw, "cmp")