Keep the log after the session that produced it

Everything this application knew about a failure went to stderr on the
desktop and to logcat on Android, and both are gone the moment the
terminal closes or the ring buffer wraps. That is fine when the person
debugging is sitting at the machine. It is useless for the case NFR-OPS-1
actually describes, and the one Android makes normal: somebody reproduces
a bug on a tablet, and then sends us a file.

A large amount of Android behaviour has never run on a device — image
intents read over JNI, an ExportProvider, a class loaded through the
activity's class loader, memory-pressure eviction, lost-root recovery —
and the single most likely failure of the lot, the activity's loader not
resolving our classes from android_main's thread, produces one line that
scrolls past. That line is now in a file, with the thread that emitted it
named beside it.

dr_plat::state answers "where does this platform keep state for this
app": $XDG_STATE_HOME/darkroom on Linux, and on Android whatever the
entry point declares. Separate from configuration and from the catalog
for the reason XDG separates them — state is the thing nobody backs up
and the user may delete without consequence.

dr_plat::diagnostics is the sink. Two files of 4 MiB, so the worst case
is a number rather than a discovery on a full phone; one line per write
with no BufWriter anywhere, because on Android processes are killed
rather than ended and a buffered log loses exactly the line it was kept
for; and redaction applied at the sink rather than at the call sites,
since a rule every author has to remember is not a rule. It tees the
platform's own logger rather than replacing it, so logcat is unchanged —
losing that while debugging would have made this a downgrade.

Android logs to external_data_path, not internal. Both are app-private
and both survive backgrounding; what separates them is that
/data/data/<pkg>/files needs run-as against a debuggable build to read
and /sdcard/Android/data/<pkg>/files is a plain adb pull from any build.
A log nobody can retrieve is not a diagnostic. The consequence is that
anyone holding the tablet can read it, which is why the redaction is
where it is, and why configuration stays on internal_data_path.

What is redacted is what NFR-SEC-2 and NFR-OPS-1 name: credentials and
tokens, found by the keyword that nearly always sits next to them, plus
the two forms that carry one with no keyword at all — an Authorization
scheme and a URL's userinfo. What is deliberately not redacted is
filesystem paths and the names of the user's photographs. They are in
neither requirement's list, and "failed to decode <redacted>" is not a
diagnostic; the preview-and-consent step NFR-OPS-1 asks for governs those
better than scrubbing would, because it lets the user look.

The over-redaction failure is tested as carefully as the under-redaction
one. A scrubber that eats "using basic sRGB as the fallback" makes a log
useless without ever being caught.

platform/dr-plat/Cargo.toml

@@ -8,7 +8,12 @@ license.workspace = true
 [dependencies]
 dr-types.workspace = true
 thiserror.workspace = true
-log.workspace = true
+# `std` is not one of `log`'s default features, and `diagnostics::install`
+# needs `set_boxed_logger`, which is gated behind it. Stated here rather than
+# left to feature unification with whichever dependant happens to pull
+# `env_logger` in — that unification holds for `cargo build --workspace` and
+# not for `cargo build -p dr-plat`.
+log = { workspace = true, features = ["std"] }
 
 [target.'cfg(all(unix, not(target_os = "android")))'.dependencies]
 keyring.workspace = true