fix(ios,macos): add -u force-undefined linker flags for @_cdecl symbols

Oracle re-review on PR #26 flagged that the Swift-side
`_ = unsafeBitCast(fn as @convention(c) ...)` static references in
ChanoraSileroSelfTest.run() are not a robust anti-dead-strip guarantee
under WMO + LTO. The optimizer can prove the discarded result has no
side effects and eliminate the address-taken reference.

The load-bearing fix is a second linker flag per symbol:

  -u _sym                forces the symbol as undefined at link time,
                         preventing the object that defines it from
                         being dropped and stopping -dead_strip from
                         removing the definition.
  -exported_symbol _sym  was already present; re-exports the symbol
                         in the binary's dynamic symbol table so the
                         Rust framework's dlsym(RTLD_DEFAULT) can find
                         it. This flag alone does NOT prevent dead-
                         strip; it only controls the export list
                         applied AFTER dead-strip.

Both flags now appear per symbol on both iOS and macOS Release
xcconfigs. The Swift-side static references stay as defense-in-depth
but are no longer the load-bearing guarantee.

apps/chanora_flutter/ios/Flutter/Release.xcconfig

@@ -2,10 +2,23 @@
 #include "Generated.xcconfig"
 
 // Force the linker to retain Swift @_cdecl symbols that the chanora_bridge
-// Rust framework resolves at runtime via dlsym(RTLD_DEFAULT). Without these
-// flags, Xcode Archive's -dead_strip removes them because no Swift caller
-// exists, and CoreML VAD silently falls back to WebRTC on TestFlight/App Store.
-OTHER_LDFLAGS = $(inherited) -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_create -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_destroy -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_reset -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_process -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_last_error -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_free_string
+// Rust framework resolves at runtime via dlsym(RTLD_DEFAULT). Two flags per
+// symbol, intentionally redundant:
+//
+//   -u _sym                marks the symbol as force-undefined at link time,
+//                          which keeps the object that defines it from being
+//                          dropped and prevents dead-strip from removing the
+//                          definition. This is the load-bearing flag.
+//   -exported_symbol _sym  re-exports the symbol in the final binary's
+//                          dynamic symbol table so dlsym(RTLD_DEFAULT) can
+//                          find it from the Rust framework at runtime.
+//
+// Without -u, Xcode Archive's -dead_strip (WMO + LTO) can remove the
+// symbol before the export list is applied, and CoreML VAD silently falls
+// back to WebRTC on TestFlight / App Store. The Swift-side static
+// `unsafeBitCast` references in SileroCoreMLBridge.swift are belt-and-
+// suspenders defense-in-depth, NOT the primary guarantee.
+OTHER_LDFLAGS = $(inherited) -Xlinker -u -Xlinker _chanora_silero_vad_create -Xlinker -u -Xlinker _chanora_silero_vad_destroy -Xlinker -u -Xlinker _chanora_silero_vad_reset -Xlinker -u -Xlinker _chanora_silero_vad_process -Xlinker -u -Xlinker _chanora_silero_vad_last_error -Xlinker -u -Xlinker _chanora_silero_vad_free_string -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_create -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_destroy -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_reset -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_process -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_last_error -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_free_string
 
 // `STRIP_STYLE = all` (Xcode default for archive installs) runs `strip` without
 // `-x`, which removes even the global @_cdecl symbols the linker exported above

apps/chanora_flutter/macos/Flutter/Flutter-Release.xcconfig

@@ -1,11 +1,11 @@
 #include? "Pods/Target Support Files/Pods-Runner/Pods-Runner.release.xcconfig"
 #include "ephemeral/Flutter-Generated.xcconfig"
 
-// macOS Release defaults to DEAD_CODE_STRIPPING = YES. Without these flags the
-// Swift @_cdecl symbols below would be stripped because no Swift caller exists;
-// the chanora_bridge Rust framework resolves them at runtime via
-// dlsym(RTLD_DEFAULT) and would silently fall back to WebRTC VAD.
-OTHER_LDFLAGS = $(inherited) -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_create -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_destroy -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_reset -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_process -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_last_error -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_free_string
+// macOS Release defaults to DEAD_CODE_STRIPPING = YES. See
+// ios/Flutter/Release.xcconfig for the full rationale; -u is the
+// load-bearing flag, -exported_symbol re-exports for dlsym, both
+// are intentionally present per @_cdecl symbol.
+OTHER_LDFLAGS = $(inherited) -Xlinker -u -Xlinker _chanora_silero_vad_create -Xlinker -u -Xlinker _chanora_silero_vad_destroy -Xlinker -u -Xlinker _chanora_silero_vad_reset -Xlinker -u -Xlinker _chanora_silero_vad_process -Xlinker -u -Xlinker _chanora_silero_vad_last_error -Xlinker -u -Xlinker _chanora_silero_vad_free_string -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_create -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_destroy -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_reset -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_process -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_last_error -Xlinker -exported_symbol -Xlinker _chanora_silero_vad_free_string
 
 // See ios/Flutter/Release.xcconfig for the STRIP_STYLE rationale.
 STRIP_STYLE = non-global