docs(v4): heat.c's and the stacks' comments say what the code does (#181)

- heat.c call_index(): the comment read as an unfinished thought ("hash-mod or
  just take low bits?", "ignore for now? Or clamp?"). It now states the rule
  the code follows: indexed by the target, a target past the table not counted,
  -1 telling the callers so; the table D-6's stand-in, which ENGINE step 4
  Part A1 plans to remove.
- stack.h points 1 and 2 said there was no overflow detection "and there never
  will be" and that a depth exceeded corrupted silently -- D-2, which D-16
  revised. Now: guarded before every opcode, a mesh node 10 and 9, the host node
  32 and 32 (D-17); C code that serves a node checks the depth itself.
- stack.c named an 8-ring and depths 10 and 9 only, and said D-2's rule still
  held.
- DATASHEET-F18.md section 13 item 4 (this gap) struck, resolved in place.

Comments only: test_stack and test_heat unchanged at both widths (730,137 and
8,272 checks, 0 failures).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017RzZgrf7V7Zk2iczBXDRbm

docs/v4.0.0/DATASHEET-F18.md

@@ -339,9 +339,11 @@ These are reported, not hidden:
    built too (`MACHINE.md` §2.2): entries 0–5, the shared math, and nothing else. In the test
    host a node Hera births carries it. The products have no births yet, so they do not link
    it. A node that is born is still sent the host's assembled system through its port.
-4. **`stack.h`'s opening comment** still says there is no overflow detection "and there never
+4. ~~**`stack.h`'s opening comment** still says there is no overflow detection "and there never
    will be". D-16 revised that, and the header carries a revision note above it. The code
-   counts and guards (§4). The comment's first paragraph is out of date.
+   counts and guards (§4). The comment's first paragraph is out of date.~~ **Resolved
+   2026-11-12 (#181):** `stack.h`'s points 1 and 2, and `stack.c`'s note, now say what the code
+   does -- counted, guarded before every opcode, the host node 32 and 32 (D-17).
 5. **Branches in slots 4 and 5** are outside the spec but not trapped (§6).
 
 ## 14. What is not in the machine