RegistryWarning: nine sibling registrars share #710's repr collision, and guard on presence rather than difference #718

#711 fixes the "overwriting X with X" message for register_protocol (#710). The same defect exists in nine sibling registrars, and in them it is worse.

The nine sites

All on main at 3904c025a, all rendering {incumbent!r} with {protocol!r}:

These reprs were added by #695. Before it, these sites interpolated only the key and could not exhibit #710.

Why it is worse here than in register_protocol

The guards differ. register_protocol tests difference:

incumbent = protocol_registry.get(name)
if incumbent is not None and incumbent is not protocol:

All nine siblings test presence:

if code in cls.__proto__:
    warn(f'protocol {code} already registered, overwriting '
         f'{cls.__proto__[code]!r} with {protocol!r}', RegistryWarning)

So a sibling emits "overwriting X with X" in two distinct situations that the message cannot tell apart:

1. Two genuinely different classes sharing a repr — the RegistryWarning claims a protocol was overwritten with itself when two distinct classes share a qualname #710 shape, where the warning is correct but unreadable.
2. Re-registering the same object, which is a no-op. Here the warning is not merely unreadable, it is misleading: nothing was displaced.

register_protocol's identity guard (added in #681) already suppresses case 2. The siblings have no equivalent.

Suggested fix

Two independent changes, in this order:

1. Add the identity guard to all nine, so a same-object re-registration stays silent. This is the behavioural half and needs its own test per site.
2. Apply fix(registry): disambiguate register_protocol's overwrite warning when two classes share a repr #711's disambiguation to the message, so case 1 stays readable once case 2 is gone.

Splitting them matters: (1) alone removes most of the confusing output, and (2) alone leaves the misleading no-op warning in place.

Provenance

Found by the cross-review of #711 at #711 (comment), which correctly identified nine siblings but cited them under pcapkit/foundation/registry/ paths that do not exist, with every line number off by one. The table above is re-derived from origin/main directly.