Clamp position precision on public / known-keys (#10665)

* Clamp position precision on public / known-keys (Compliance)

* Fix review comments: bounds check, all-channel precision clamp, disabled channel guard

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Refactor position precision comments for clarity and update channel configuration handling in AdminModule

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

src/mesh/Channels.h

@@ -86,6 +86,9 @@ class Channels
     // Returns true if the channel has the default name and PSK
     bool isDefaultChannel(ChannelIndex chIndex);
 
+    // Returns true if this channel's effective key is publicly decryptable (open or well-known/default PSK).
+    bool usesPublicKey(ChannelIndex chIndex);
+
     // Returns true if we can be reached via a channel with the default settings given a region and modem preset
     bool hasDefaultChannel();
 
@@ -144,6 +147,9 @@ extern Channels channels;
 static const uint8_t defaultpsk[] = {0xd4, 0xf1, 0xbb, 0x3a, 0x20, 0x29, 0x07, 0x59,
                                      0xf0, 0xbc, 0xff, 0xab, 0xcf, 0x4e, 0x69, 0x01};
 
+/// True if a getKey()-resolved key offers no privacy: length 0 (off) or the public defaultpsk family. Pure; for tests.
+bool cryptoKeyIsPublic(const CryptoKey &key);
+
 static const uint8_t eventpsk[] = {0x38, 0x4b, 0xbc, 0xc0, 0x1d, 0xc0, 0x22, 0xd1, 0x81, 0xbf, 0x36,
                                    0xb8, 0x61, 0x21, 0xe1, 0xfb, 0x96, 0xb7, 0x2e, 0x55, 0xbf, 0x74,
                                    0x22, 0x7e, 0x9d, 0x6a, 0xfb, 0x48, 0xd6, 0x4c, 0xb1, 0xa1};