M5b plan: task 04, PGP private key blocks too

The private-key rule also refuses a line that starts `-----BEGIN` and
ends `PRIVATE KEY BLOCK-----`, OpenPGP's armor header (RFC 9580
section 6.2.1; RFC 4880 section 6.2). The two endings are a given
table, PRIVATE_KEY_ENDS; the skeleton's private_key comment names it.
A PGP private key block joins the attack corpus, a PGP public key
block and a signature the code-work corpus, and the edge test gains an
indented PGP header (refused), a PGP public header and a PGP header
without its closing dashes (both pass).

Checks: the reference passes the gate on titan, 997 tests (unchanged:
the cases are inside existing tests), /tmp unchanged around the run;
the task's edits applied to a fresh ref-03 copy are byte-identical to
the reference; 8 private-key mutations, 4 of them new for PGP, all
caught; tag and private-key-header greps over the task and given files
are empty; given files pass rustfmt and the 500-line limit.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/plans/M5b/04-sanitize-refuse.md

@@ -39,7 +39,7 @@ character is refused by `control`.
 | 7 | `secret-sk` | `sk-` keys | `what looks like an API key` |
 | 8 | `secret-slack` | `xoxb-`, `xoxa-`, `xoxp-`, `xoxr-`, `xoxs-` tokens | `what looks like a Slack token` |
 | 9 | `secret-aws` | `AKIA` or `ASIA` access key ids | `what looks like an AWS access key` |
-| 10 | `secret-private-key` | a line that starts a private-key block | `a private key` |
+| 10 | `secret-private-key` | a line that starts a private-key block: PEM, OpenSSH or OpenPGP | `a private key` |
 | 11 | `tag` | any listed tag, matched exactly as `neutralise` matches it | `chat-template markup` |
 
 The table, the sentences and every list of characters and prefixes are **given, filled in**, in the
@@ -89,16 +89,18 @@ the new file. That is expected: do not touch `crates/memoryd`.
   and one `ghs_` form, `secret.mixed` is true: the run must also hold an ASCII upper-case letter
   and an ASCII digit, because names like `sk-folding-cube-spinner` start the same way. `find_secret`
   (given) walks the prefixes and calls your `starts_word` and `key_run`.
-- **A private key**: a line that, white space aside, starts `-----BEGIN` and ends
-  `PRIVATE KEY-----`. A public key or a certificate ends differently and passes; so does the header
-  in the middle of a sentence.
+- **A private key**: a line that, white space aside, starts `-----BEGIN` and ends with one of
+  `PRIVATE_KEY_ENDS` (given): `PRIVATE KEY-----` (PEM and OpenSSH) or `PRIVATE KEY BLOCK-----`
+  (OpenPGP's armor header, RFC 9580 section 6.2.1). A public key, a PGP public key block, a
+  signature or a certificate ends differently and passes; so does a header in the middle of a
+  sentence.
 - **The tag rule** is task 03's matching, reused: `next_tag` in `tags.rs` finds the first listed
   tag exactly as `neutralise` does. Edit E2 lets `refuse.rs` call it. This is narrower than M5a's
   markup rule, which matched pieces of tags in any case: an upper-case tag is not the model's token
   (measured in task 03), and a tag split by a zero-width character is refused by `invisible`.
 - **Given code you must not change**: the `RULES` table (its closures turn into `fn` pointers), the
-  `Secret` tables, `find_secret` (its `if let … &&` chain is Rust 2024; it compiles on this
-  toolchain), `invisible`, `refuse` and the key-character functions.
+  `Secret` tables, `PRIVATE_KEY_ENDS`, `find_secret` (its `if let … &&` chain is Rust 2024; it
+  compiles on this toolchain), `invisible`, `refuse` and the key-character functions.
 - The calls you need, all in the standard library:
 
   | Call | Signature, and what it gives |
@@ -117,7 +119,7 @@ the new file. That is expected: do not touch `crates/memoryd`.
   | `str::split_at_checked` | `fn split_at_checked(&self, mid: usize) -> Option<(&str, &str)>` |
   | `str::rsplit`, `str::split`, `Iterator::next` | `before.rsplit('\n').next()`: the last line piece; `from.split('\n').next()`: the first |
   | `Option::unwrap_or_default` | `Option<&str>` to `&str`, `""` for `None` (not `unwrap`: library code never panics) |
-  | `str::trim`, `str::trim_end`, `str::is_empty`, `str::ends_with` | white space off both ends, off the end; `s.ends_with("PRIVATE KEY-----")` |
+  | `str::trim`, `str::trim_end`, `str::is_empty`, `str::ends_with` | white space off both ends, off the end; `line.ends_with(end)` |
   | `next_tag` (in `tags.rs`) | `fn next_tag(text: &str) -> Option<(&str, &'static Tag, &str)>`: before, tag, after |
 
 ## The edits, exactly
@@ -182,9 +184,9 @@ Nothing else in `tags.rs` changes: not its comment, not its body.
   ```
 
   The command of step 2 now compiles, with warnings about unused variables, an unused import,
-  `PRIVATE_USE` and the fields of `Secret`: they are the unfilled functions, and go as you fill
-  them. In `sanitize_refuse` 14 tests fail, in `sanitize_refuse_corpus` 2; every failure is
-  `not yet implemented`.
+  `PRIVATE_USE`, `PRIVATE_KEY_ENDS` and the fields of `Secret`: they are the unfilled functions,
+  and go as you fill them. In `sanitize_refuse` 14 tests fail, in `sanitize_refuse_corpus` 2;
+  every failure is `not yet implemented`.
 - [ ] **4. Fill the eight functions, one at a time, in this order**, each from its comment, and
   run `cargo check -p bxcore` after each: `control`, `first_char`, `private_use`, `noncharacter`,
   `listed_tag`, `starts_word`, `key_run`, `private_key`. Change nothing else in `refuse.rs`.