test: one disc resolver, no machine-specific defaults, all three corpora in the container

Finishes #16 in the three places its earlier remedies missed.

`tests/`: the last four local `disc_root()` copies now use `tests/common`, and
with them goes the one real hardcoded fallback — `ui_keyframe_record_disc.rs`
fell back to an absolute path on one machine, which made `unset SYLPHEED_DISC`
a no-op there. Control: with the corpus absent that suite now finishes in 0.00s
instead of 57.55s, so it skips rather than finding a disc of its own.

`examples/`: seventeen examples defaulted to `/disc`, the mount point inside the
CI container. Redundant there — `docker/ci/run` sets `SYLPHEED_DISC=/disc` — and
wrong everywhere else, where a missing corpus turned into a file-not-found
against a path that has never existed on the host. They now name the variable to
set, like the other hundred examples already did.

`docker/ci/run`: mount `$SYLPHEED_RES3D` and `$SYLPHEED_ISO` alongside the disc.
Only the disc was mounted, so an in-container run sat out the res3d and iso
suites while looking like a full one — the defect this issue is about, in the
runner itself.

Measured in the container on this desktop with all three corpora present:
45 suites / 377 passed / 0 failed / 14 ignored, and `sylpheed-corpus-report.txt`
now reports PRESENT for all three rather than for the disc alone.

Refs #16.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/sylpheed-export/examples/bank_chunks.rs

@@ -7,7 +7,8 @@ use std::process::Command;
 use sylpheed_formats::media;
 
 fn main() {
-    let disc = std::env::var("SYLPHEED_DISC").unwrap_or_else(|_| "/disc".into());
+    let disc =
+        std::env::var("SYLPHEED_DISC").expect("set SYLPHEED_DISC to the extracted disc root");
     let src = media::DirectorySource::new(&disc);
     for bank in ["BGM_103.slb", "BGM_102.slb", "BGM_001.slb"] {
         match media::sound_bank_riffs(&src, bank) {

crates/sylpheed-export/examples/bgm_size_census.rs

@@ -18,7 +18,8 @@
 use sylpheed_formats::media;
 
 fn main() {
-    let root = std::env::var("SYLPHEED_DISC").unwrap_or_else(|_| "/disc".into());
+    let root =
+        std::env::var("SYLPHEED_DISC").expect("set SYLPHEED_DISC to the extracted disc root");
     let src = media::DirectorySource::new(&root);
     const WANT: [usize; 2] = [3_876_864, 3_930_112];
     let (mut found, mut matches) = (0usize, Vec::new());

crates/sylpheed-export/examples/dialog_pairs.rs

@@ -20,7 +20,8 @@ use std::collections::BTreeSet;
 use sylpheed_formats::{pak, ratc, ui_layout};
 
 fn main() {
-    let root = std::env::var("SYLPHEED_DISC").unwrap_or_else(|_| "/disc".into());
+    let root =
+        std::env::var("SYLPHEED_DISC").expect("set SYLPHEED_DISC to the extracted disc root");
     let ar = pak::PakArchive::open(format!("{root}/dat/GP_DIALOG.pak")).expect("GP_DIALOG.pak");
     let sets: Vec<Option<BTreeSet<String>>> = ar
         .entries()

crates/sylpheed-export/examples/dialog_rows.rs

@@ -17,7 +17,8 @@
 use sylpheed_formats::{pak, ratc, ui_layout};
 
 fn main() {
-    let root = std::env::var("SYLPHEED_DISC").unwrap_or_else(|_| "/disc".into());
+    let root =
+        std::env::var("SYLPHEED_DISC").expect("set SYLPHEED_DISC to the extracted disc root");
     // 🔴 WIDENED 2026-08-31 to every pak, to check the Decoder's rival search
     // independently. They report zero four-button builds within 6 px of
     // 259/329/399/469 anywhere on the disc, which turns "another dialog with

crates/sylpheed-export/examples/rat_leaf.rs

@@ -8,7 +8,8 @@
 use sylpheed_formats::{pak::PakArchive, ui_layout};
 
 fn main() {
-    let disc = std::env::var("SYLPHEED_DISC").unwrap_or_else(|_| "/disc".into());
+    let disc =
+        std::env::var("SYLPHEED_DISC").expect("set SYLPHEED_DISC to the extracted disc root");
     let ar = PakArchive::open(format!("{disc}/dat/GP_TITLE.pak")).expect("open");
     let e = &ar.entries()[4]; // entry 4 = the English title
     let bundle = ar.read(e).expect("read");

crates/sylpheed-export/examples/record_loop_control.rs

@@ -48,7 +48,8 @@ fn main() {
         .unwrap_or(8);
     unsafe { OFFSET = off };
     println!("  reading the loop length at header +0x{off:02x}");
-    let root = std::env::var("SYLPHEED_DISC").unwrap_or_else(|_| "/disc".into());
+    let root =
+        std::env::var("SYLPHEED_DISC").expect("set SYLPHEED_DISC to the extracted disc root");
     let mut paks: Vec<_> = std::fs::read_dir(format!("{root}/dat"))
         .expect("dat/")
         .flatten()

crates/sylpheed-export/examples/record_population.rs

@@ -11,7 +11,8 @@
 use sylpheed_formats::{pak, ratc, ui_layout};
 
 fn main() {
-    let root = std::env::var("SYLPHEED_DISC").unwrap_or_else(|_| "/disc".into());
+    let root =
+        std::env::var("SYLPHEED_DISC").expect("set SYLPHEED_DISC to the extracted disc root");
     let mut paks: Vec<_> = std::fs::read_dir(format!("{root}/dat"))
         .expect("dat/")
         .flatten()

crates/sylpheed-export/examples/static_with_cycle.rs

@@ -11,7 +11,8 @@
 use sylpheed_formats::{pak, ratc, ui_layout};
 
 fn main() {
-    let root = std::env::var("SYLPHEED_DISC").unwrap_or_else(|_| "/disc".into());
+    let root =
+        std::env::var("SYLPHEED_DISC").expect("set SYLPHEED_DISC to the extracted disc root");
     let ar = pak::PakArchive::open(format!("{root}/dat/GP_TITLE.pak")).expect("GP_TITLE.pak");
     let (mut total, mut hits, mut multipose) = (0usize, 0usize, 0usize);
     for (i, e) in ar.entries().iter().enumerate() {

crates/sylpheed-export/examples/voice_chunks.rs

@@ -7,7 +7,8 @@ use std::process::Command;
 use sylpheed_formats::{media, slb::VoiceLang};
 
 fn main() {
-    let disc = std::env::var("SYLPHEED_DISC").unwrap_or_else(|_| "/disc".into());
+    let disc =
+        std::env::var("SYLPHEED_DISC").expect("set SYLPHEED_DISC to the extracted disc root");
     let src = media::DirectorySource::new(&disc);
     for movie in ["ADV", "S00A", "RT01A"] {
         let Some((s, e)) = media::resolve_movie_voice_region(&src, movie, VoiceLang::English)