polish(fetch,mcp): robots parser + firefox client cache + Acquire ordering (P3) (#23)

Three P3 items from the 2026-04-16 audit. Bump to 0.3.17.

webclaw-fetch/sitemap.rs: parse_robots_txt used trimmed[..8] slice
plus eq_ignore_ascii_case for the directive test. That was fragile:
"Sitemap :" (space before colon) fell through silently, inline
"# ..." comments leaked into the URL, and a line with no URL at all
returned an empty string. Rewritten to split on the first colon,
match any-case "sitemap" as the directive name, strip comments, and
require `://` in the value. +7 unit tests cover case variants,
space-before-colon, comments, empty values, non-URL values, and
non-sitemap directives.

webclaw-fetch/crawler.rs: is_cancelled uses Ordering::Acquire
instead of Relaxed. Behaviourally equivalent on current hardware for
single-word atomic loads, but the explicit ordering documents intent
for readers + compilers.

webclaw-mcp/server.rs: add lazy OnceLock cache for the Firefox
FetchClient. Tool calls that repeatedly request the firefox profile
without cookies used to build a fresh reqwest pool + TLS stack per
call. Chrome (default) already used the long-lived field; Random is
per-call by design; cookie-bearing requests still build ad-hoc since
the cookie header is part of the client shape.

Tests: 85 webclaw-fetch (was 78, +7 new sitemap), 272 webclaw-core,
43 webclaw-llm, 11 CLI — all green. Clippy clean across workspace.

Refs: docs/AUDIT-2026-04-16.md P3 section

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.toml

@@ -3,7 +3,7 @@ resolver = "2"
 members = ["crates/*"]
 
 [workspace.package]
-version = "0.3.16"
+version = "0.3.17"
 edition = "2024"
 license = "AGPL-3.0"
 repository = "https://github.com/0xMassi/webclaw"