unlock BOOT_LOCK for h4 SYS region

H4 has no BTPG/BTER equivalents from V3 — BOOT_LOCK in STATR gates SYS
access instead. unlock_boot() writes KEY1/KEY2 to BOOT_MODEKEYR to clear
it; standard PER/FTPG then operates on 0x1FFF0000 directly.

probe-rs still won't attach the sys algo until ch32-data flips SYS_1
write:false → true.

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

Author: aq1018

algos/h4/src/bin/sys.rs

@@ -23,7 +23,7 @@ algorithm!(Algo, {
 
 impl FlashAlgorithm for Algo {
     fn new(_addr: u32, _clock: u32, _function: Function) -> Result<Self, ErrorCode> {
-        unlock_main();
+        unlock_boot();
         Ok(Self)
     }
 
@@ -44,6 +44,6 @@ impl FlashAlgorithm for Algo {
 
 impl Drop for Algo {
     fn drop(&mut self) {
-        lock_main();
+        lock_boot();
     }
 }

algos/h4/src/lib.rs

@@ -53,6 +53,19 @@ pub fn unlock_options() {
     FLASH.obkeyr().write(|w| w.set_obtkey(KEY2));
 }
 
+/// BOOT_LOCK gates SYS access; standard PER/FTPG flow then targets 0x1FFF0000
+/// directly (H4 has no BTPG/BTER equivalents).
+pub fn unlock_boot() {
+    unlock_main();
+    FLASH.boot_modekeyr().write(|w| w.set_bootkeyr(KEY1));
+    FLASH.boot_modekeyr().write(|w| w.set_bootkeyr(KEY2));
+}
+
+pub fn lock_boot() {
+    FLASH.statr().modify(|w| w.set_boot_lock(true));
+    lock_main();
+}
+
 pub fn page_erase(addr: u32) {
     wait_busy();
     FLASH.ctlr().modify(|w| w.set_per(true));