fix(native): pad minidump_context_arm_t to match on-disk minidump spec

minidump_context_arm_t was 72 bytes (integers + cpsr), but the
on-disk minidump spec for ARM contexts — as implemented by breakpad
(MDRawContextARM) and rust-minidump (CONTEXT_ARM, the reader Sentry's
ingest uses to deserialize these streams) — is 368 bytes, with a
trailing float-save area (fpscr u64, 32x u64 VFP regs, 8x u32 extra).
rust-minidump's struct is explicitly called out as a "Breakpad
extension":

  https://github.com/rust-minidump/rust-minidump/blob/d4fefc765aad35b3bef569d53c1680eadab5a268/minidump-common/src/format.rs#L1034-L1055

Since get_context_size() returns sizeof(minidump_context_arm_t), the
stream size recorded in the minidump was too small and downstream
parsers couldn't deserialize the thread context.

Add the missing float-save trailing fields. They're left zero-
initialized (we don't capture VFP state yet — same punt the i386
branch makes for FPU) but the layout now matches what readers expect.
The other arches in this file (i386, aarch64) already carry their
full structs this way.

Note: crashpad's in-memory MinidumpContextARM is 364 bytes (u32
fpscr), a 4-byte divergence from breakpad despite crashpad's comment
saying it's "included for compatibility with breakpad." That's a
crashpad-internal quirk; on-disk minidumps follow the breakpad layout
that rust-minidump reads.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jpnurmi

src/backends/native/minidump/sentry_minidump_format.h

@@ -304,10 +304,17 @@ PACKED_STRUCT_BEGIN
 typedef struct {
     uint32_t context_flags;
     uint32_t r[13]; // R0-R12
-    uint32_t sp;
-    uint32_t lr;
-    uint32_t pc;
+    uint32_t sp; // R13
+    uint32_t lr; // R14
+    uint32_t pc; // R15
     uint32_t cpsr;
+    // VFP/NEON state (matches breakpad's MDRawContextARM / Microsoft's
+    // CONTEXT_ARM floating-point area). We don't populate these today
+    // but the layout needs to match what downstream minidump parsers
+    // (crashpad, rust-minidump, breakpad) expect when reading the stream.
+    uint64_t fpscr;
+    uint64_t fpregs[32]; // D0-D31
+    uint32_t fpextra[8];
 } PACKED_ATTR minidump_context_arm_t;
 PACKED_STRUCT_END
 #endif