hive-forge: name the CI job ci-log actually served

`ci-log --run <n> --job <idx>` accepted any index and exited 0. Past the
run's job count it printed job 0's log, with nothing marking the
substitution. Measured against run 3363 (4 jobs): indices 0-3 gave three
distinct md5s, while 99 and 12345 both returned output byte-identical to
job 0.

Both web log routes clamp an out-of-range index to job 0 and answer 200.
I had claimed nothing in the response distinguishes them -- that was
measured on the body only, and the body was the wrong place to look. The
`Content-Disposition` header names the job the server actually served:

    --job 3   -> filename="ci-doc-pointer lint-8363.log"
    --job 99  -> filename="ci-nix flake check-8360.log"   (job 0's)

So the fix is to report what came back rather than to pre-validate the
index. Establishing the real job count needs two extra API calls on every
indexed read -- `ActionRun` carries no job count, and the only job
endpoints are the per-run listing and a per-job log keyed by internal job
id, not by index. The header costs nothing: it is already in the response
being read.

The provenance line goes to stderr, so it cannot corrupt a piped log;
under `--json` it is a `served` field instead. An out-of-range `--job` is
therefore no longer an error -- it is a read whose true subject is named.

`get_bytes_named` is a sibling of `get_bytes_raw` rather than a signature
change, leaving the attachment and artifact download paths untouched.

Rebased onto main after the two flat-rename PRs landed; the only conflict
was the test module's import list, resolved by keeping both sides. While
reading the surrounding context this commit's own defect surfaced:
`check_status`'s doc comment had been left glued to the head of
`disposition_filename`'s, so one function carried two unrelated
descriptions and the other carried none. No gate can see that -- it is
well-formed rustdoc either way.

hive-forge/src/client.rs

@@ -10,7 +10,9 @@ use std::path::PathBuf;
 use anyhow::{Context, Result, anyhow, bail};
 use forgejo_api::{Auth, ForgejoError};
 use reqwest::blocking::{Client as HttpClient, Response};
-use reqwest::header::{ACCEPT, AUTHORIZATION, CONTENT_TYPE, HeaderMap, HeaderValue};
+use reqwest::header::{
+    ACCEPT, AUTHORIZATION, CONTENT_DISPOSITION, CONTENT_TYPE, HeaderMap, HeaderValue,
+};
 use serde::{Deserialize, Serialize};
 use serde_json::Value;
 
@@ -218,10 +220,30 @@ impl Client {
     /// attachment downloads. Uses the full URL as-is; the caller is
     /// responsible for constructing it (see `attachment_url`).
     pub fn get_bytes_raw(&self, url: &str) -> Result<Vec<u8>> {
+        self.get_bytes_named(url).map(|(bytes, _)| bytes)
+    }
+
+    /// Like [`Client::get_bytes_raw`], but also returns the filename the
+    /// server put in `Content-Disposition`.
+    ///
+    /// Forgejo's persisted Actions-log route names the download after the
+    /// job it actually served, and that header is the **only** part of the
+    /// response identifying which job came back: the route clamps an
+    /// out-of-range job index to job 0 and answers 200, so the body alone
+    /// is byte-identical to a legitimate read.
+    ///
+    /// # Errors
+    /// Same as [`Client::get_bytes_raw`] — transport failure or a non-2xx.
+    pub fn get_bytes_named(&self, url: &str) -> Result<(Vec<u8>, Option<String>)> {
         let resp = self.web.get(url).send().context("GET")?;
         let resp = check_status(resp, &format!("GET {url}"))?;
+        let name = resp
+            .headers()
+            .get(CONTENT_DISPOSITION)
+            .and_then(|v| v.to_str().ok())
+            .and_then(disposition_filename);
         resp.bytes()
-            .map(|b| b.to_vec())
+            .map(|b| (b.to_vec(), name))
             .with_context(|| format!("read bytes for GET {url}"))
     }
 
@@ -449,6 +471,18 @@ fn read_token() -> Result<String> {
     Ok(raw.trim().to_owned())
 }
 
+/// The quoted `filename="…"` out of a `Content-Disposition` value.
+///
+/// Deliberately reads the plain `filename=` parameter and not the RFC 5987
+/// `filename*=UTF-8''…` one that Forgejo sends alongside it: the starred
+/// form is percent-encoded, so it needs decoding before it can be shown,
+/// and the plain form carries the same name.
+fn disposition_filename(value: &str) -> Option<String> {
+    let after = value.split_once("filename=\"")?.1;
+    let (name, _) = after.split_once('"')?;
+    (!name.is_empty()).then(|| name.to_owned())
+}
+
 /// Surface non-2xx HTTP responses on the raw web routes as anyhow
 /// errors with the response body included (matches `curl
 /// --fail-with-body`) — turns silent failures into errors with a
@@ -464,7 +498,35 @@ fn check_status(resp: Response, op: &str) -> Result<Response> {
 
 #[cfg(test)]
 mod tests {
-    use super::{index, owner_repo_from_git_url, split_repo};
+    use super::{disposition_filename, index, owner_repo_from_git_url, split_repo};
+
+    #[test]
+    fn disposition_filename_reads_forgejos_real_header() {
+        // Verbatim from a persisted Actions-log download, both parameters
+        // present in the order Forgejo emits them.
+        let v = "attachment; filename=\"ci-doc-pointer lint-8363.log\"; \
+                 filename*=UTF-8''ci-doc-pointer%20lint-8363.log";
+        assert_eq!(
+            disposition_filename(v).as_deref(),
+            Some("ci-doc-pointer lint-8363.log")
+        );
+    }
+
+    #[test]
+    fn disposition_filename_absent_or_unusable() {
+        assert_eq!(disposition_filename("attachment"), None);
+        assert_eq!(disposition_filename(""), None);
+        // Only the starred form: not decoded here, so it must not be
+        // mistaken for a usable plain filename.
+        assert_eq!(
+            disposition_filename("attachment; filename*=UTF-8''a%20b.log"),
+            None
+        );
+        // Unterminated quote — must not return a truncated name.
+        assert_eq!(disposition_filename("attachment; filename=\"oops"), None);
+        // Present but empty.
+        assert_eq!(disposition_filename("attachment; filename=\"\""), None);
+    }
 
     #[test]
     fn owner_repo_from_git_url_strips_scheme_and_creds() {