re: not ONE of the 80 forced instances has a file-read key

The port agent pointed out that forced_backdrop_necessity.rs collapsed
sprite_layer_key (a u16 read from the T8aD header, decoded) with
implied_layer_key (this crate's table of positions MEASURED in the running
game), and that 'has its own key' therefore reads as file-backed when it is
not. Splitting them is stronger than either of us stated:

    read from the T8aD header:   0
    implied (measured):         14   10x pfbase.tbm, 4x palogo_eff0.prm
    nothing at all:             66   62 decided, 4 inert

Zero. There is no instance on the disc where a forced element also carries a
file-read layer key, so this rule has never been checked against a decoded
field -- there is no case where both can speak. That is what a keyless-element
fallback necessarily looks like, but it removes a check a reader would assume
exists.

Also corrects something I said to the port and had wrong. 'None of the 18 is
evidence for the rule in any direction' conflated two questions. Whether the
rule changes the composite: no, the sort already had the key. Whether the rule
gets the RIGHT answer: yes, and the 14 implied keys are measured positions, so
this is the rule agreeing with the oracle -- its only external corroboration,
and there are 14 instances of it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Wuu56cE8vJGTBtn1ppsk8v

crates/sylpheed-formats/examples/forced_backdrop_key_source.rs

@@ -0,0 +1,71 @@
+//! Of the forced instances the rule merely CONFIRMS, how many have a key READ
+//! FROM THE FILE, and how many an IMPLIED key that is itself a measurement?
+//!
+//! `forced_backdrop_necessity.rs` asked only whether an element had *a* key,
+//! collapsing `sprite_layer_key` (a `u16` read out of the `T8aD` header — decoded)
+//! with `implied_layer_key` (this crate's per-name table of positions **measured
+//! in the running game**). For counting whether the rule moves anything that is
+//! the right question. For describing what a confirmation is *made of*, it is not:
+//! "the file already settles it" and "another measurement already settles it" are
+//! different claims, and a reader who sees "own key" will take the first.
+//!
+//! Raised by the port agent 2026-08-30.
+//!
+//!     cargo run -p sylpheed-formats --example forced_backdrop_key_source
+
+use std::path::PathBuf;
+
+use sylpheed_formats::{pak::PakArchive, ui_layout};
+
+fn main() {
+    let root = PathBuf::from(std::env::var("SYLPHEED_DISC").expect("SYLPHEED_DISC"));
+    let mut paks: Vec<PathBuf> = std::fs::read_dir(root.join("dat"))
+        .expect("dat/")
+        .filter_map(|e| e.ok().map(|e| e.path()))
+        .filter(|p| p.extension().is_some_and(|x| x == "pak"))
+        .collect();
+    paks.sort();
+
+    let (mut read, mut implied, mut none) = (0usize, 0usize, 0usize);
+    println!("# archive  entry  element  key_source  key");
+    for pak in &paks {
+        let Ok(ar) = PakArchive::open(pak) else { continue };
+        let name = pak.file_name().unwrap().to_string_lossy().to_string();
+        for (i, e) in ar.entries().iter().enumerate() {
+            let Ok(by) = ar.read(e) else { continue };
+            let Some(b) = ui_layout::parse_build(&by) else {
+                continue;
+            };
+            for el in &b.elements {
+                if !ui_layout::forced_backdrop(&b, el) {
+                    continue;
+                }
+                let (src, key) = match ui_layout::sprite_layer_key(&b, &by, el) {
+                    Some(k) => {
+                        read += 1;
+                        ("read_T8aD", Some(k))
+                    }
+                    None => match ui_layout::implied_layer_key(&el.name) {
+                        Some(k) => {
+                            implied += 1;
+                            ("implied_MEASURED", Some(k))
+                        }
+                        None => {
+                            none += 1;
+                            ("none", None)
+                        }
+                    },
+                };
+                println!(
+                    "  {name}  {i}  {}  {src}  {}",
+                    el.name,
+                    key.map(|k| format!("0x{k:08X}")).unwrap_or("-".into())
+                );
+            }
+        }
+    }
+    println!("\n# forced instances by key source:");
+    println!("#   read from the T8aD header (decoded):        {read}");
+    println!("#   implied — this crate's MEASURED name table: {implied}");
+    println!("#   none — only forced_backdrop can speak:      {none}");
+}