fix(asm): materialise large RISC-V immediates

Assisted-by: DeepSeek V4 Pro

CHANGELOG.md

@@ -61,6 +61,13 @@ Unreleased changes on the `development` branch.
   `C.LDSP`/`C.SDSP`), plus `C.ADDI4SPN`, `C.ADDW` and `C.SUBW`.  The
   byte-exact ground-truth test exercises these against `GOARCH=riscv64
   go tool asm`.
+- **RISC-V large-immediate materialisation.**  `ADDI`/`ANDI`/`ORI`/`XORI`
+  with a 32-bit immediate that does not fit 12 bits now expand exactly as
+  `cmd/asm`: two `ADDI`s for the small `ADDI` split range, and
+  `LUI`+`ADDIW`+`<op>` otherwise, with the `LUI` and `ADDIW` compressed to
+  `C.LUI`/`C.ADDIW` when their immediate fits six signed bits.  The
+  byte-exact ground-truth test covers positive, negative, and out-of-range
+  immediates against `GOARCH=riscv64 go tool asm`.
 - **Debugger watchpoint slots.**  `gasm debug`'s `watch` command always used
   hardware watchpoint slot 0, so a second `watch` call silently overwrote
   the first.  Watchpoint slots are now tracked in the `Session` (DR0–DR3);