Remove dead logic, duplicated helpers, and style slop

[0x0bug]

What changed

Mechanical clean-code fixes. No intended production behavior changes. Invalid synthetic snapshots with null parent entries now return a structured failure instead of no result; the only user-visible production difference is normalized label spacing in report output (three misaligned columns).

Dead logic removed

• Test-WdtTuiUnicodeLogoSupport computed a Windows Terminal signal and then ignored it in a tautological expression ($utf8 -and ($wt -or $utf8) ≡ $utf8). The dead signal and its two parameters are removed from the detection chain (Test-WdtTuiUnicodeLogoSupport, Get-WdtTuiLogoModeDecision, Get-WdtTuiLogoMode). Documented behavior — Unicode logo on interactive UTF-8 output — is unchanged (README "Unicode and ASCII logo modes", docs/usage.md) and is now pinned by direct tests.
• The trailing return in Test-WdtSnapshotMembership (process-runner) was removed as unreachable, which was true for the production path — snapshot entries are always non-null pscustomobjects, so the loop can only exit through in-loop returns. Re-review found the function is also called directly with synthetic snapshots (tests), where a parent key present with a $null value would have ended the walk and returned nothing. The parent lookup is now guarded explicitly (ContainsKey + null check → SnapshotParentMissing), keeping the function total without reintroducing a dead-looking trailing line, with a regression test that fails without the guard.
• Test-WdtAllowedW32tmProcessShape accepted -LocalFunctionNames and never read it; Test-WdtAllowedNewObjectCommand and Get-WdtMemberSafetyIssue carried the parameter only to forward it. The whole forwarding chain is removed. Get-WdtCommandSafetyIssue keeps its parameter — it genuinely uses it.
• An assignment whose value was immediately discarded before continue in the TUI resize path now discards explicitly ($null =).

Duplication removed

• Group-EventLogEvents reimplemented the events module's own ConvertTo-OneLineMessage inline (same 240/237 truncation constants). It now calls the helper. tests/diagnostic-classification.tests.ps1 imports the helper alongside the function under test — the isolated-import harness was why the inline copy existed in the first place.

Consistency

• Protect-WdtRegexMatches assigned to the automatic $matches variable; renamed to $regexMatches. This is hygiene only — modules run in separate child processes, so no cross-contamination was possible. (The same pattern exists in scripts/bootstrap/run.ps1:13, intentionally not fixed here: the bootstrap is under a byte-stability contract with the published site copy and the SHA-256 pinned in tests/bootstrap-sync.tests.ps1; it needs a coordinated WDT-change → site-sync pass.)
• $approvalInventory / $ApprovalInventory casing unified in the services module.
• Report label columns re-aligned: Pending states (Suspicious) (services), Samples (performance), findings-summary hashtable in report-common.

Regression tests added

• Test-WdtSnapshotMembership with a parent key mapped to $null returns structured SnapshotParentMissing (verified to fail without the guard).
• Get-WdtTuiLogoModeDecision contract: overrides, redirected output, UTF-8 vs OEM encodings (6 cases).
• ConvertTo-OneLineMessage through Group-EventLogEvents: whitespace-only → No message; >240 chars → 240 with ...; newline collapse.

Verification

• scripts/validate.ps1: pass on PowerShell 7 and Windows PowerShell 5.1 (0 parser / 0 safety / 0 layout / 0 generated).
• Full test suite (19 files × both engines): all pass.
• Live smoke: -Module Events,Services,Performance -PrivacyMode → exit 0, Collection completeness: Complete, event messages one-lined and truncated by the shared helper, aligned labels confirmed in the generated report.
• git diff --check: clean.

Not touched

scripts/bootstrap/run.ps1 (byte-stability contract), the +4 menu-count coupling in Get-WdtTuiVisibleIndexes and cross-module helper consolidation (Write-Section/Format-Bytes/ConvertTo-SafeSingleLine → report-common) — those need AST-allowlist changes and deserve their own PR.

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