fix(output): 19 quadratic regex runs on the output path, worst ~5.7h at the cap

The output gate claimed LLM10 self-safety on the grounds that its patterns have
no nested quantifiers. True, and irrelevant: nesting is not what makes these
blow up. A run in front of a REQUIRED literal, reachable from a short anchor, is
enough -- crafted input repeats the anchor and never supplies the literal, so
every start position rescans the tail. Quadratic, not exponential, and the
max_scan_chars cap does not help: it bounds the input, and quadratic work on a
bounded input is still hours.

Measured, not argued. `<a:` x 100_000 took 23.4s in AUTOLINK_RE alone; the
composed gate on that payload took 458.7s, extrapolating to ~5.7 hours at the
1_000_000-char input the gate itself accepts. Size-matched ordinary prose runs
0.31s, so the separation is 18x-660x -- unlike the blob in the neighbouring
test, which is the *faster* side of prose and never exercised backtracking.

Two fixes, chosen per pattern rather than uniformly:

- active_content + lexicon JSON (15 runs): exclude the character that opens the
  pattern's own anchor (`[` for markdown, `<` for tags), so a run cannot reach
  past the next start position and the per-start costs telescope. Verified to
  cost no recall: long URLs, long alt text, and `<` inside a quoted attribute
  all still match. Bounding instead would have been linear too but wrong here --
  the content is attacker-controlled, so padding past a bound would be a
  one-line bypass of the EchoLeak class this table exists to catch.
- connstr egress (4 runs): bound the password at MAX_CONNSTR_VALUE. The
  exclusion fix is unavailable -- the anchor character is `/` and passwords
  containing `/` are the common case (measured: they match today). The residual
  miss is a credential over 256 chars; a token that long is still caught by
  egress:jwt-token.

hybrid-xss:script-tag had neither option: its run is the script BODY, which may
legitimately contain `<`. It now matches the opening tag and drops the
`</script>` requirement. That also closes a fail-open -- `<script>alert(1)`
unclosed was silently missed -- at the cost of flagging prose that merely
mentions `<script>`, now documented.

Found by the composed-gate test staying red after every individual scanner was
already linear: the lexicon's six html-obfuscation patterns were the remaining
813x. A per-scanner test alone would have shipped that.

662 passed (was 642), and faster than before the fix.

src/llm_ingestion_guard/calibration.py

@@ -40,10 +40,25 @@ ENTROPY_HEX_FLOOR_LEN = 64
 
 # --- lexicon: self-safety + variant thresholds ------------------------------
 # Input-size cap (OWASP LLM10): large enough for a real ingested document;
-# beyond it the scanner reads the prefix and flags, so runtime stays bounded
-# even on a decompression-bomb-sized input.
+# beyond it the scanner reads the prefix and flags, so every sub-scanner sees a
+# bounded input. Note what this cap does NOT buy: bounded input is only bounded
+# runtime if the patterns are linear in it. A quadratic pattern turns this cap
+# into hours of work, which is what crafted input against the output path was
+# measured to do before the ReDoS fix (see active_content's pattern-table note).
 MAX_SCAN_CHARS = 1_000_000
 
+# --- output: secret-egress self-safety --------------------------------------
+# Longest password a connection-string pattern will match. A bound is required
+# (not merely nice) because the run sits in front of a mandatory `@`: unbounded,
+# crafted input repeating `redis://:` and never supplying the `@` makes every
+# start position rescan the tail — quadratic. Excluding the anchor character the
+# way the active-content table does is not available here, since that character
+# is `/` and passwords containing `/` are the common case.
+# 256 is generous for a password and cheap to scan; the residual miss is a
+# credential longer than this, which for the realistic case (a token used as a
+# DB password) is still caught by the jwt-token / high-specificity patterns.
+MAX_CONNSTR_VALUE = 256
+
 # Minimum length before the rot13 variant is scanned — shorter strings hit
 # rot13-look-alike false positives.
 ROT13_MIN_LEN = 40