docs: make a custom System One base URL obvious

[codeitlikemiley]

Callers could already override the host, but docs and examples hid it. Reject a blank or non-http URL at build, add a mock example, and record mock-HTTP bench numbers. No hot-path change.

[codeitlikemiley]

Comment. No code change requested.

The override works. normalize_base_url runs once inside Config::resolve at build. system_one and models both use format!("{}{path}", config.base_url). The only src/ change is src/config.rs. Auth, JSON body, retry, and timeout stay as they are when build succeeds.

Checked on 9a17634 with rustc 1.98.1. cargo test --lib config:: passed (trailing slash and whitespace, blank, ftp, relative). custom_base_url_serves_system_one_and_models and same_origin_307_is_followed passed with --features mock.

Not blocking.

• A base URL that contains ? or # is accepted. The join is still string append, so /v1/systemone lands in the query or the fragment. That join is unchanged from main. Rejecting those strings at build would be new policy.
• docs/audit-2026-09-24.md says the example locks a path prefix. The contract test does (/gateway/v1/...). examples/custom_base_url.rs mounts /v1/systemone and /v1/models on the wiremock origin.
• examples/README.md now says live mode uses TYPESAFE_BASE_URL or else https://api.typesafe.ai. custom_base_url returns Error::Sdk when that variable is unset. The other examples still fall back.
• Common failure 10 says the base URL is the server root. README and the contract test also allow a path prefix such as /gateway.
• The runnable example does not call .model(). The README and AGENTS snippets do. Live mode therefore sends jev-latest.
• The audit says no example called .base_url before this PR. examples/support/harness.rs and examples/blocking_triage.rs already did, against wiremock.
• Bench numbers in the audit were not reproduced here. The harness binds wiremock on 127.0.0.1, disables retries, and does not read TYPESAFE_API_KEY. CI compiles the bench and does not run it.
• An accepted http or https URL still matches the Python 0.6.0 string join. Rust now rejects an empty or non-http(s) base at build. Python 0.6.0 only does rstrip("/").

No Bugbot threads. New module-doc comments are not a reason to change the code.