build_site_audit.build() has no test: the notes refusal, commit-date conversion, CLAW entry and sort are unchecked

[realmarcin]

From the round-1 review of #265 (#238).

Where: scripts/fleet/build_site_audit.py build(), and SiteAuditBuilderTests.

Every new test calls build_entry() or committed_candidates(). None calls build(), which derives:

• each Mech's UTC commit date;
• the whole CLAW entry;
• checked_at_utc, local_date, pinned_at_utc and scope;
• the repository order;
• the refusal when notes are missing.

A mutation run disabling the notes refusal, the UTC conversion, the CLAW entry and the sort still passed all 82 tests. #238 asks that the tests cover how each mechanical field is derived.

Fix: add an offline test of build() with fake pins, notes, template, stats, fetch and git show.