feat(asm): add AArch64 arm64 encoder with ground-truth verification

Assisted-by: MiMo V2.5 Pro

docs/ARCHITECTURE.md

@@ -212,6 +212,17 @@ relocations).  Like the RISC-V encoder it is validated byte-for-byte against
 `GOARCH=loong64 go tool asm`, and its GOOBJ output is proven end-to-end by
 substituting it into a cross-compiled `go build` and linking with `cmd/link`.
 
+An **AArch64 encoder** (Phase 5, arm64) encodes the integer instruction set
+with the data-processing (shifted register and immediate forms), load/store
+(scaled unsigned immediate and unscaled9-bit immediate), conditional and
+unconditional branches, the MOV pseudo-instruction and its constant
+materialisation (MOVZ/MOVN/MOVK for wide immediates, ORR with logical bitmask
+encoding for values like `$1`), the FP/SP frame mapping (autosize =
+align16(frame+8), prologue using pre-index store for small frames and
+STP+SUB for large frames) and SB/global symbol references (ADRP+ADD pairs with
+R_ADDRARM64 relocations).  Like the other encoders it is validated
+byte-for-byte against `GOARCH=arm64 go tool asm`.
+
 On top of the encoder, `Assemble` walks a parsed `TEXT` body, converts each
 operand to an encoder operand, and lays the instructions out so local labels
 resolve to relative jump offsets: jumps start in the short (rel8) form and