hrw4u: Fix u4wrh cidr() emitting 1-arg form missing IPv6 bits

[Clendenin]

Summary

HRW %{CIDR:a} supplies only IPv4 bits and relies on implicit IPv6 defaults,
while Prism cidr() requires both ipv4_bits and ipv6_bits. When u4wrh sees
the single value form, pad with the canonical HRW IPv6 default (48) so the
emitted Prism has valid arity and round-trips stably.

This change also extends the CIDR reverse branch to handle the bare %{CIDR}
no-payload form, padding to cidr(24, 48) using HRW's canonical defaults so
the forward round-trip satisfies the 2-arg signature.

[Copilot]

Pull request overview

Fixes a u4wrh (reverse compiler) bug where HRW %{CIDR} and %{CIDR:N} forms produced HRW4U cidr() calls with too few arguments, violating the 2-arg requirement enforced by forward validation. The reverse path now pads to HRW's canonical defaults (v4=24, v6=48), restoring stable round-tripping.

Changes:

• Add _pad_cidr_args helper and a CIDR-specific branch in InverseSymbolResolver.percent_to_ident_or_func to fill in defaults for missing IPv4/IPv6 bit values.
• Add reverse-only test fixtures for bare %{CIDR}, single-arg %{CIDR:N}, and v6-only %{CIDR:,N} forms.
• Mark new fixtures as u4wrh-only exceptions since forward emission always produces explicit two-arg %{CIDR:v4,v6}.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

File	Description
tools/hrw4u/src/hrw_symbols.py	Adds _pad_cidr_args and CIDR special case in reverse function tag handling.
tools/hrw4u/tests/data/conds/exceptions.txt	Registers cidr-single and cidr-bare as u4wrh-only fixtures.
tools/hrw4u/tests/data/conds/cidr-single.{input,output,ast}.txt	New fixture covering %{CIDR:16}, %{CIDR:24,64}, %{CIDR:,8} reverse cases.
tools/hrw4u/tests/data/conds/cidr-bare.{input,output,ast}.txt	New fixture covering bare %{CIDR} reverse case.