re(ui): the 60 nameless RATC children are frames, not children -- .tan decoded

Closes the reach caveat the `opt ` name fix left behind: 60 of 18 002 RATC
children carry no `opt ` block, and it was not established whether they lack one
or sit past our 128-byte window.

Neither. They are not children. `examples/ratc_optless_children.rs` re-runs
`ratc::parse`'s own guards over the disc and reports which one fired: all 60 are
"tag beyond the window", none is rejected by length, gap or charset, none is
child #0, and all 60 live in six bundles of one archive. Within a bundle the
distances back to the nearest tag are an exact arithmetic progression, step
60 600 -- ten different records finding the SAME tag, because there is only one.

Reading a bundle directly: children 1..10 are equal-size T8aD blocks under a
single `opt ` name, `pb_f15_eg_anm.tan`. `.tan` is a FRAME SEQUENCE. One block
declares the resource; its payload is a run of T8aD frames.

Disc-wide, over all 18 718 `opt ` names in all 33 paks: a RATC bundle names
exactly six kinds of resource -- `.t32` 14 756, `.rat` 3 311, `.prm` 367,
`.tbm` 224, `.sbo` 54, `.tan` 6. Six `.tan`, ten frames each = 60, the entire
population with nothing left over. The negative is closed, not narrowed.

Consequence recorded but deliberately not fixed: `ratc::parse` over-reports
there, listing a `.tan`'s frames as anonymous children. Nothing in the menu
milestone reads a `.tan` -- it occurs only in GP_READY_ROOM, which S1 ruled
out -- so no screen the port draws changes.

Also a METHOD entry for this container OOM-killing `slb_leading_segment_disc`
under default test parallelism (SIGKILL, no assertion; 8/8 pass with
--test-threads=1).

crates/sylpheed-formats/examples/ratc_optless_children.rs

@@ -0,0 +1,119 @@
+//! The 60 RATC children that carry no `opt ` block — do they lack it, or is our
+//! window too small?
+//!
+//! [`ratc::parse`] now prefers the name a child's own `opt ` block states, and
+//! falls back to the old backwards printable-run scan when there is no block
+//! within 128 bytes. That fallback fires for 60 of the disc's 18 002 children
+//! (0.3 %), and the reach of the finding in `docs/re/structures/ratc-child-names.md`
+//! stops there: "whether they genuinely lack the block or sit past the search
+//! window is not established".
+//!
+//! This settles that. For every child with no accepted block it reports
+//!
+//!   * whether an `opt ` tag exists at all further back, and how far;
+//!   * which guard rejected a tag that WAS in the window (length, gap, charset);
+//!   * the child's position in its bundle and its magic, in case the opt-less
+//!     ones are structurally distinct (e.g. always the first child);
+//!   * the raw bytes before the magic, so the fallback's answer can be judged.
+//!
+//!     cargo run -p sylpheed-formats --example ratc_optless_children -- <pak>...
+use sylpheed_formats::{pak, ratc};
+
+/// Why a child has no accepted `opt ` name. Mirrors `ratc::opt_name`'s guards
+/// one for one, so a rejection here is the same rejection the parser made.
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Clone)]
+enum Why {
+    /// No `opt ` tag in the 128-byte window, and none anywhere before it either.
+    NoTagAtAll,
+    /// No tag in the window, but one exists further back, this many bytes away.
+    TagBeyondWindow(usize),
+    /// Tag found, but its BE32 length is 0 or > 64.
+    BadLength(usize),
+    /// Tag found, name ends more than 8 bytes before the magic — a neighbour's.
+    GapTooBig(usize),
+    /// Tag found, the named bytes are not all printable ASCII.
+    NotGraphic,
+}
+
+/// The parser's own window.
+const WINDOW: usize = 128;
+
+fn classify(buf: &[u8], at: usize) -> Option<Why> {
+    let lo = at.saturating_sub(WINDOW);
+    let pos = match buf[lo..at].windows(4).rposition(|w| w == b"opt ") {
+        Some(p) => lo + p,
+        None => {
+            // Widen to the whole buffer before the child: is it merely far away?
+            return Some(match buf[..at].windows(4).rposition(|w| w == b"opt ") {
+                Some(p) => Why::TagBeyondWindow(at - p),
+                None => Why::NoTagAtAll,
+            });
+        }
+    };
+    let len = u32::from_be_bytes(buf.get(pos + 4..pos + 8)?.try_into().ok()?) as usize;
+    if len == 0 || len > 64 || pos + 8 + len > at {
+        return Some(Why::BadLength(len));
+    }
+    let gap = at - (pos + 8 + len);
+    if gap > 8 {
+        return Some(Why::GapTooBig(gap));
+    }
+    let s = String::from_utf8_lossy(&buf[pos + 8..pos + 8 + len]);
+    if s.is_empty() || !s.chars().all(|c| c.is_ascii_graphic()) {
+        return Some(Why::NotGraphic);
+    }
+    None // accepted — this child is not one of the 60
+}
+
+fn main() {
+    let mut total = 0usize;
+    let mut rows: Vec<(String, usize, usize, String, String, Why)> = Vec::new();
+    let mut first_child_of_bundle = 0usize;
+    for path in std::env::args().skip(1) {
+        let Ok(ar) = pak::PakArchive::open(&path) else { continue };
+        let short = path.rsplit('/').next().unwrap_or(&path).to_string();
+        let entries: Vec<_> = ar.entries().to_vec();
+        for (ei, e) in entries.iter().enumerate() {
+            let Ok(bytes) = ar.read(e) else { continue };
+            let Some(kids) = ratc::parse(&bytes) else { continue };
+            for (ci, c) in kids.iter().enumerate() {
+                total += 1;
+                let Some(why) = classify(&bytes, c.offset) else { continue };
+                if ci == 0 {
+                    first_child_of_bundle += 1;
+                }
+                let lo = c.offset.saturating_sub(24);
+                let hex = bytes[lo..c.offset]
+                    .iter()
+                    .map(|b| format!("{b:02x}"))
+                    .collect::<Vec<_>>()
+                    .join(" ");
+                rows.push((short.clone(), ei, ci, c.name.clone(), hex, why));
+            }
+        }
+    }
+    println!("RATC children scanned          : {total}");
+    println!("  with NO accepted `opt ` block: {}", rows.len());
+    println!("  ... of which are child #0    : {first_child_of_bundle}");
+
+    let mut by_why: std::collections::BTreeMap<String, usize> = Default::default();
+    for r in &rows {
+        let k = match &r.5 {
+            Why::TagBeyondWindow(_) => "TagBeyondWindow".to_string(),
+            Why::BadLength(_) => "BadLength".to_string(),
+            Why::GapTooBig(_) => "GapTooBig".to_string(),
+            other => format!("{other:?}"),
+        };
+        *by_why.entry(k).or_default() += 1;
+    }
+    println!("\nwhy, by cause:");
+    for (k, n) in &by_why {
+        println!("   {k:20} x{n}");
+    }
+
+    println!("\nevery occurrence (name is what the FALLBACK scan returned):");
+    for (p, ei, ci, name, hex, why) in &rows {
+        println!("   {p:28} entry {ei:4} child {ci:3}  {name:24} {why:?}");
+        println!("      24 bytes before the magic: {hex}");
+    }
+}

crates/sylpheed-formats/src/ratc.rs

@@ -110,7 +110,12 @@ fn opt_name(bytes: &[u8], off: usize) -> Option<String> {
 /// between the name and the magic, so an exact-adjacency scan isn't enough.
 ///
 /// Fallback only -- [`opt_name`] is the stated name. 60 of the disc's 18 002
-/// children have no `opt ` block and still rely on this.
+/// children have no `opt ` block and still rely on this, and all 60 are
+/// accounted for: they are the ten frames of the disc's only `.tan` frame
+/// sequence, in six language copies of one `GP_READY_ROOM` bundle, where a
+/// single `opt ` block names the whole run. See
+/// `docs/re/structures/ratc-tan-frame-sequence.md` -- and note that this means
+/// `parse` OVER-reports there, listing frames as children.
 fn name_before(bytes: &[u8], off: usize) -> String {
     let start = off.saturating_sub(96);
     let window = &bytes[start..off];