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

Assisted-by: MiMo V2.5 Pro

CHANGELOG.md

@@ -9,6 +9,19 @@ and this project adheres to [Conventional Commits](https://www.conventionalcommi
 
 Unreleased changes on the `development` branch.
 
+### Added
+
+- **arm64 encoder (Phase 5 — complete).**  `gasm asm` can now assemble `_arm64.s`
+  files: the AArch64 integer instruction set with the MOV pseudo-instruction and
+  its immediate-constant expansions (MOVZ/MOVN/MOVK for wide immediates, ORR with
+  logical bitmask encoding for values like `$1`), data-processing (shifted
+  register and immediate forms), load/store (scaled unsigned and unscaled9-bit
+  immediate), conditional and unconditional branches, FP/SP frame mapping,
+  SB/global symbol references (ADRP+ADD pairs with `R_ADDRARM64` relocations),
+  jump chain folding, and ELF64 emission (`gasm asm --format elf`).  Ground-truth
+  verification against `GOARCH=arm64 go tool asm` matches byte-for-byte.  Phase 5
+  (the other architectures — RISC-V, LoongArch, arm64) is now complete.
+
 ## [0.30.0] — 2026-08-13
 
 The LoongArch encoder (Phase 5) ships with ELF64 and GOOBJ emission, verified