bindings: what a book says about itself, and what the register says back

book-pipeline reaches for colophon and bibfetch on every book it repairs.
Shelling out would mean a process per book per tool across five thousand
books, and parsing back the JSON it is about to parse anyway.

bibfetch's fetch loop lived in main.rs, so binding it meant either
lifting it or writing a second one. A second implementation of 'which
register answers about this kind of identifier, and in what order' would
drift from the first, and the copy nobody runs from a terminal would
drift silently. It is now bibfetch::ask, and the command line calls it
too — verified by running the CLI before and after and diffing.

fetch_identifier releases the GIL around the request. It is the only
function here that opens a socket, and holding the interpreter for the
length of an HTTP call would stop every other thread in it.

bibfetch is optional and only the python feature turns it on: its HTTP
client does not build for wasm32, and an unconditional dependency would
have taken the JavaScript package down with it.

crates/bindings/Cargo.toml

@@ -30,6 +30,7 @@ crate-type = ["cdylib", "rlib"]
 [dependencies]
 booksniff.workspace = true
 bibclean.workspace = true
+colophon.workspace = true
 chunkmd.workspace = true
 citefind.workspace = true
 corpus-core.workspace = true
@@ -47,6 +48,11 @@ textkit.workspace = true
 
 pyo3 = { workspace = true, optional = true }
 
+# Only the Python build gets this. bibfetch is the one tool here that opens a
+# socket, and its HTTP client does not build for wasm32 — so a dependency that
+# is unconditional would take the JavaScript package down with it.
+bibfetch = { workspace = true, optional = true }
+
 # The JavaScript build. Only ever compiled for a wasm32 target; on any other
 # the module is compiled out and these sit unused.
 wasm-bindgen = { workspace = true, optional = true }
@@ -56,7 +62,7 @@ serde-wasm-bindgen = { workspace = true, optional = true }
 # Only maturin turns this on. pyo3's extension-module feature leaves the
 # Python symbols to be resolved by the interpreter at load time, so an
 # ordinary `cargo build` would fail to link if it were on by default.
-python = ["dep:pyo3"]
+python = ["dep:pyo3", "dep:bibfetch"]
 # The C ABI. Off by default so an ordinary build does not also export a
 # hundred symbols nobody asked for.
 capi = []