docs(rfcs): add anti-speculative-implementation conventions guidance

When merging PR #2105 (Hermes Run Adapter RFC) the standing concern was
that landing the RFC unconfirmed would invite the speculative-fragment
implementation pattern we just had to put on hold with PR #2071 — well-
written 651-LOC standalone scripts with no callers.

Add a single bullet to the conventions block so the contract is explicit:
an RFC is a design direction, not an invitation to PR fragments against
it. Implementation slices need maintainer confirmation first.

Applied during stage-341 build, not requested from @Michaelyklam — the
guardrail belongs in the conventions doc itself rather than as a one-off
ask on this PR.

docs/rfcs/README.md

@@ -18,6 +18,12 @@ cutting infrastructure.
   questions, Rollout plan. Skip what doesn't apply.
 - An RFC is a starting point for review. Comments and revisions land via PR
   edits, not separate discussion threads.
+- An RFC documents a design direction. It is **not** an invitation to file
+  implementation PRs against fragments of it. Before opening any PR that
+  implements an accepted RFC, confirm with a maintainer in the tracking
+  issue that the implementation slice is wanted and that no other
+  contributor is already building it. Speculative implementations of RFC
+  fragments without a confirmed integration site will be held.
 
 ## When to file an RFC