add --dump-section=BASE:LEN:PATH for end-of-run guest memory snapshot

Drives byte-level memory diffs against canary's Memory::Save dump.
Hot-path zero-cost when absent; lockstep digest unaffected
(instructions=100000003 deterministic across reruns).

crates/xenia-app/src/main.rs

@@ -221,6 +221,13 @@ enum Commands {
         /// `XENIA_MEM_WATCH`. Example: `--mem-watch=0x828F40B4`.
         #[arg(long)]
         mem_watch: Option<String>,
+        /// Diagnostic. Dump a contiguous guest memory range to a file at
+        /// end-of-run. Format: `BASE:LEN:PATH` (BASE/LEN hex or decimal).
+        /// Example: `--dump-section=0x80000000:0x10000000:v80.bin`.
+        /// Read via `GuestMemory::read_bulk`; uncommitted pages read as
+        /// zero. Read-only; lockstep digest unaffected.
+        #[arg(long)]
+        dump_section: Option<String>,
     },
     /// Browse XISO disc image contents
     Browse {
@@ -380,6 +387,7 @@ fn main() -> Result<()> {
             dump_addr,
             branch_probe,
             mem_watch,
+            dump_section,
         } => cmd_exec(
             &path,
             max_instructions,
@@ -402,6 +410,7 @@ fn main() -> Result<()> {
             dump_addr.as_deref(),
             branch_probe.as_deref(),
             mem_watch.as_deref(),
+            dump_section.as_deref(),
         ),
         Commands::Browse { path } => cmd_browse(&path),
         Commands::Info { path } => cmd_info(&path),
@@ -607,6 +616,7 @@ fn cmd_exec(
     dump_addr: Option<&str>,
     branch_probe: Option<&str>,
     mem_watch: Option<&str>,
+    dump_section: Option<&str>,
 ) -> Result<()> {
     cmd_exec_inner(
         path,
@@ -630,6 +640,7 @@ fn cmd_exec(
         dump_addr,
         branch_probe,
         mem_watch,
+        dump_section,
         None,
         None,
         false,
@@ -672,6 +683,7 @@ fn cmd_check(
         None, // dump_addr — same
         None, // branch_probe — diagnostic, never wanted on goldens
         None, // mem_watch — same
+        None, // dump_section — same
         out,
         expect,
         stable_digest,
@@ -700,6 +712,7 @@ fn cmd_exec_inner(
     dump_addr: Option<&str>,
     branch_probe: Option<&str>,
     mem_watch: Option<&str>,
+    dump_section: Option<&str>,
     digest_out: Option<&str>,
     digest_expect: Option<&str>,
     stable_digest: bool,
@@ -1107,6 +1120,37 @@ fn cmd_exec_inner(
         }
     }
 
+    let dump_section_combined: Option<String> = match (dump_section, std::env::var("XENIA_DUMP_SECTION").ok()) {
+        (Some(s), _) => Some(s.to_string()),
+        (None, Some(s)) if !s.is_empty() => Some(s),
+        _ => None,
+    };
+    if let Some(spec) = dump_section_combined {
+        let parts: Vec<&str> = spec.splitn(3, ':').collect();
+        if parts.len() != 3 {
+            return Err(anyhow::anyhow!(
+                "--dump-section expects BASE:LEN:PATH, got {spec:?}"
+            ));
+        }
+        let parse_u32 = |t: &str| -> Result<u32> {
+            if let Some(h) = t.strip_prefix("0x").or_else(|| t.strip_prefix("0X")) {
+                u32::from_str_radix(h, 16).map_err(|e| anyhow::anyhow!("bad hex {t:?}: {e}"))
+            } else {
+                t.parse::<u32>().map_err(|e| anyhow::anyhow!("bad dec {t:?}: {e}"))
+            }
+        };
+        let base = parse_u32(parts[0].trim())?;
+        let len = parse_u32(parts[1].trim())?;
+        let path = std::path::PathBuf::from(parts[2].trim());
+        if !quiet {
+            tracing::info!(
+                "dump-section armed: base={:#010x} len={:#x} path={}",
+                base, len, path.display()
+            );
+        }
+        kernel.dump_section = Some((base, len, path));
+    }
+
     // Install the GPU register aperture MMIO region on the guest memory so
     // any `0x7FC8xxxx` access routes to our atomic mailbox. Matches canary's
     // `graphics_system.cc:141-144`. The callbacks capture Arc clones of the
@@ -3707,6 +3751,27 @@ fn dump_thread_diagnostic(
             }
         }
     }
+
+    if let Some((base, len, ref path)) = kernel.dump_section {
+        let mut buf = vec![0u8; len as usize];
+        const CHUNK: usize = 4096;
+        let mut off = 0usize;
+        while off < buf.len() {
+            let take = CHUNK.min(buf.len() - off);
+            mem.read_bulk(base.wrapping_add(off as u32), &mut buf[off..off + take]);
+            off += take;
+        }
+        match std::fs::write(path, &buf) {
+            Ok(()) => println!(
+                "\n=== dump-section: wrote {} bytes from {:#010x} to {} ===",
+                buf.len(), base, path.display(),
+            ),
+            Err(e) => eprintln!(
+                "dump-section: failed to write {}: {e}",
+                path.display(),
+            ),
+        }
+    }
 }
 
 #[allow(clippy::too_many_arguments)]