Put the developer docs under docs/dev and index the folder for users first

docs/ had 26 developer documents flat beside the manual, and the two
audiences are very differently sized: most readers want the manual and
the gesture reference, a few want the register, the designs and the
measurements. The manual and gestures.md stay at the top; everything for
someone changing the code moves to docs/dev/, and the two documents that
name their own successors — the v0.1 milestone and the UI-refinement plan
— go to docs/dev/archive/ rather than being deleted, since both are still
cited. docs/README.md is the index, users first.

Every reference follows: code comments, Cargo manifests, the workflows,
the pre-commit hook, the bench and traceability tools (which locate the
repo root by docs/dev/requirements.md now), packaging, the Docker READMEs,
CLAUDE.md, CONTRIBUTING.md and the README. The matrix links one level
deeper and is regenerated. Links out of the moved documents into the tree
gain a level; a link checker over every Markdown file finds none broken.

tools/bench/src/main.rs

@@ -1,4 +1,4 @@
-//! The benchmark suite `docs/requirements.md` §8 has been promising.
+//! The benchmark suite `docs/dev/requirements.md` §8 has been promising.
 //!
 //! §8 says performance is verified by *"an automated benchmark suite against a
 //! synthetic 50k catalog, run per-commit … A regression beyond stated
@@ -191,17 +191,17 @@ enum Mode {
 fn print_help() {
     println!(
         "\
-dr-bench — DarkRoom's performance suite (docs/requirements.md §8)
+dr-bench — DarkRoom's performance suite (docs/dev/requirements.md §8)
 
   run       measure and print, judging nothing
   check     measure, print, and exit 1 on a violated budget or a regression
-  record    measure and rewrite docs/bench-baseline.json from the result
+  record    measure and rewrite docs/dev/bench-baseline.json from the result
 
 Flags:
   --reference        this machine is the reference desktop, so machine-sensitive
                      budgets are asserted rather than reported
   --fixture <dir>    where the synthetic 50k catalog lives (or $DR_BENCH_DIR)
-  --baseline <file>  the committed numbers (default docs/bench-baseline.json)
+  --baseline <file>  the committed numbers (default docs/dev/bench-baseline.json)
   --lanes <n>        sweep lanes for the thumbnail row (default: CPU threads)
   --thumbnails <n>   images in the thumbnail row (default 1200)
 
@@ -307,8 +307,8 @@ fn measure_and_report(
     }
     println!();
     println!(
-        "  {} gate(s) failed. docs/benchmarks.md says what each metric measures and\n  \
-         docs/bench-baseline.json holds the numbers these used to be.",
+        "  {} gate(s) failed. docs/dev/benchmarks.md says what each metric measures and\n  \
+         docs/dev/bench-baseline.json holds the numbers these used to be.",
         failures.len()
     );
     Ok(mode != Mode::Gate)
@@ -453,7 +453,7 @@ fn print_verdict(
     machine: &str,
     comparable_fixture: bool,
 ) -> Vec<String> {
-    println!("Against docs/bench-baseline.json");
+    println!("Against docs/dev/bench-baseline.json");
     match (&base.recorded_on, cx.same_machine) {
         (None, _) => println!(
             "  No baseline has been recorded yet. Budgets are still gated; drift is not.\n  \