Ground action for all weapons; forbid diving attacks (G.44)

[kohutovic]

Targets readability-refactor (PR #13), not master, since #13 is still open.

Why: a scope gap the refactor left behind

The 2.0.0-draft CHANGELOG records that "universal forbidden actions [were]
promoted out of the longsword-only close-quarter section". G.38–G.43 were
promoted. G.47 (ground action) and G.49 (falls, dropped weapons) were not.

Both sit under ## Close quarter combat, whose preamble reads "allowed only
where the weapon-specific rules permit them (currently only in the Longsword
category)". The consequence: in sabre, rapier and sword & buckler there is no
article covering a fencer going to the ground. A referee has only the general
G.8(e) halt trigger to lean on.

This looks like an incomplete step of the refactor rather than a deliberate
scoping decision.

What changed

1. New article G.44 — a rules change, not a fix:

A fencer must not attack by jumping, diving, or otherwise throwing the body
in a manner that does not end with the fencer on both feet and in control.

Penalised under 1.10 (first group: yellow, then red; hit annulled), or as a
dangerous action under 2.4 where it endangers the opponent. An involuntary
loss of balance is explicitly excluded — that stays with the falls article.

Intent to put on record: the wording deliberately leaves the flèche /
pass-through permitted — the fencer ends on their feet and in control, merely
in motion. What it forbids is the dive, the knee-drop and the knee-slide. If the
intent is to also restrict the flèche, the criterion would have to be about
crossing the legs, not about the landing, and that is a different article.

2. G.47 and G.49 promoted out of the longsword-only section into a new
## Falls and ground action (all weapons), completing what the refactor began.
No change to their text. Articles G.45–G.51 renumbered; only two external
references existed (penalty rows 1.10 and 4.1), both updated.

3. Identifier stability clause amended — flagged here so it is not
discovered in the diff. The introduction claimed "Once assigned, an identifier
never changes", which the renumbering violated. Those identifiers were
introduced in this same unreleased draft and have never been published, so the
guarantee protected nothing while forbidding structural fixes before release.
The clause now binds from the first published version, and the CHANGELOG
records that this is the last permitted renumbering.

Kept as a separate commit so it can be approved or questioned independently of
the rule change.

Verification

• scripts/check_conventions.sh and scripts/check_links.sh pass.
• Chapter G: 51 articles, contiguous, no duplicates, in order.
• No stale references to the old G.44–G.50 numbering anywhere in the repo.
• PDF regenerated: the new heading resolves in the TOC to p. 19 and the heading
  is on p. 19; G.44 (p. 19), G.51 (p. 20) and penalty row 1.10 (p. 53) all
  render as expected.

Suggested follow-up (not in this PR)

check_conventions.sh validates wording but not article sequence or
cross-references — the checks above were ad hoc. Now that identifier stability
is a stated rule of the document, a sequence/xref check would be worth adding.