Add stage-3 content triage: sort unknown files into binary/ and text/

Unknown files are no longer terminal. Stage 1 now routes :unknown onto a
dedicated queue (with the same blocking backpressure as the known queue),
and a third worker pool sorts each file into data/binary/ or data/text/
using a NUL-byte sniff of the first 8000 bytes.

- content.jl: is_binary content sniff (stage 3)
- worker.jl: handle_unknown_job; stage-1 routes unknown with backpressure;
  KNOWN_ENQUEUE_RETRY_SECONDS -> ROUTE_ENQUEUE_RETRY_SECONDS (serves both)
- config.jl: unknown_worker_count/queue_capacity, binary_dir, text_dir + env
- FileServer.jl: unknown queue, pool, stage-aware recovery, drain ordering
- tests for is_binary and handle_unknown_job; tmp_config isolates new dirs
- README: three-stage pipeline

src/content.jl

@@ -0,0 +1,24 @@
+# Stage-3 content triage for unknown files.
+#
+# A file that stage-1 couldn't recognize is still sorted into one of two coarse
+# buckets so downstream tooling can treat them differently: `text/` for
+# human-readable content, `binary/` for everything else. The test is the classic
+# "NUL byte in the first sniff window" heuristic that git and file(1) use — cheap
+# (no full read), and reliable in practice: text encodings don't embed NUL bytes,
+# while binary formats almost always do near the start.
+
+const CONTENT_SNIFF_BYTES = 8000
+
+"""
+    is_binary(path) -> Bool
+
+Classify a file as binary (`true`) or text (`false`) by sniffing its first
+`CONTENT_SNIFF_BYTES` bytes for a NUL byte. An empty file has no NUL, so it is
+treated as text.
+"""
+function is_binary(path::AbstractString)::Bool
+    open(path, "r") do io
+        chunk = read(io, CONTENT_SNIFF_BYTES)
+        return any(==(0x00), chunk)
+    end
+end