test: add comprehensive tests to improve coverage

[Copilot]

Summary

Analyzes and improves test coverage across the codebase. Before this PR, overall statement coverage was 10.5% with 6 packages at 0%.

Changes

Bug fix

• internal/athena/pos_test.go — Corrected a wrong test expectation: the test expected /123 to not match commandRegex, but the regex (^/[a-z0-9]+(-[a-z0-9]+)*) and its own code comment explicitly allow numeric commands like /51. This caused a pre-existing test failure.

New test files

File	Before	After
internal/sliceutil/sliceutil_test.go	0%	100%
internal/permissions/permissions_test.go	0%	100%
internal/packet/packet_test.go	0%	100%
internal/playercount/playercount_test.go	0%	100%
internal/uidheap/uidheap_test.go	0%	100%
internal/webhook/webhook_extra_test.go	4.7%	62.5%
internal/area/area_extra_test.go	21.1%	53.4%
internal/settings/settings_test.go	0%	42.6%

What each test file covers

• sliceutil — ContainsString / ContainsInt with found, not-found, empty, nil, and edge-value cases
• permissions — GetPermissions (empty, single, multi-perm, admin), HasPermission (exact, superset, missing, admin), IsModerator (no-perm, CM-only, kick, admin)
• packet — NewPacket (header-only, with body, trailing-# stripping, empty/whitespace header errors), String (no body, with body, empty field), roundtrip through network-layer stripping
• playercount — GetPlayerCount, AddPlayer, RemovePlayer, symmetric add/remove, concurrent add/remove with -race
• uidheap — Push/Pop, Less, Swap, Len, min-heap ordering verification
• webhook — empty-URL early-returns for all 6 post functions, HTTP error response, negative-UID "N/A" rendering
• area — HP bounds/validation, Status, Lock, Background, Buffer ring-rotation, Reset, PlayerCount/VisiblePlayerCount, LastSpeaker, SpectateInvited, SpectateMode clearing invite list, ClearInvited, SwapEvidence out-of-bounds, Taken helper, SwitchChar-to-spectator, EvidenceMode get/set
• settings — DefaultConfig field spot-checks, LoadFile success/missing/empty, GetConfig with TOML overrides and missing file

Overall coverage: 10.5% → 12.5%

All tests pass with -race.