feat(lenses): the commit-truth lens, repo-facts, as Gate 4

Every claim in every commit message, and in the documentation the range
adds or changes, proven or gone. The lens's block kinds are text, doc and
slot: a message unit is keyed on its text, the base and the code up to its
commit (MessageDeps); a documentation unit on its file, its normalised
text, the base and the code of the whole range (DocKeyParts); a diffstat on
its text alone; whole:order and whole:absent are asked in slot passes of
its own, never stored. A documentation file's diff header and a part the
commit writes nothing in are a mechanical n/a. Budgets 60/40 for a text or
doc block, 50/35 for a slot; one read. Gate 4, after the proofs, so the
messages and documents are read once the code they describe has settled;
the owner's gate is 5.

The goldens that follow from the lens table are derived by
tests/Goldens/derive.py, which asserts that the recorded table still
reproduces the recorded entries before it writes.

Proven, with the opt-in commit after this one: the suite (26 passed); a dry
run of a scratch range (2 commits, a README with sections) by the previous
binary and this one with the three static lenses on and the rest off, 25
prompts and 4 system files byte for byte; with repo-facts on as well,
those files unchanged and 10 passes and 1 system file added.
The lens's firing proof in this pipeline is owed (the owner's go).

interfaces/Crafter.Review-Prompt.cppm

@@ -23,6 +23,13 @@ export namespace Crafter::Review::Prompt {
     // KIND_HINTS_REPO
     extern const std::string_view HintOutside;
     extern const std::string_view HintAbsent;
+    // The block hints of the commit-truth lens (2026-10-31): a message unit,
+    // and a unit of a documentation file.
+    extern const std::string_view TextBlockHintRepo;
+    extern const std::string_view DocBlockHintRepo;
+    // SLOT_DUTIES: what a slot pass of a lens covers beyond the slot's
+    // question; empty for every other lens and slot.
+    std::string_view SlotDuty(std::string_view lensFile, std::string_view unitId);
 
     // _common-repo.md with the artifact kind filled in.
     std::string SystemText();