[CmdPal] Time-decayed frecency signal for main-page ranking

[michaeljolley]

Phase 2 of the CmdPal search overhaul (stacked)

Base branch is dev/mjolley/cmdpal-search-overhaul (phase 1), NOT main. Phase 3 (provider weighting) will stack on this branch.

What changed

Replaces the list-position frecency heuristic in RecentCommandsManager with a real time-decayed signal so recently- and frequently-used commands reorder correctly within their ranker tier. The tier always dominates; frecency only reorders within a tier, so overall ordering behavior is preserved.

• Time-decay weight: recency = 2^(-ageDays / halfLife) combined with log2(uses + 1) for frequency, clamped to the previous ~0..70 range so the ranker's within-tier math and FrecencyScale = 1.0 are unaffected.
• HistoryItem.LastUsed: new persisted DateTimeOffset driving the decay.
• Store cap grown from 50 to 500 entries (frecency decays gracefully, so a larger store is cheap and improves recall of older-but-frequent commands).
• Legacy migration: persisted items without LastUsed deserialize as MinValue and are treated as used one day ago, so day-one behavior degrades to Uses-ordering rather than all-equal.
• Serialization fix: the internal History property was never emitted by the source-generated serializer, so recent-command history was silently not persisted across restarts. Marked it [JsonInclude] so history (and the new LastUsed) actually round-trips. Old empty persisted state still deserializes cleanly.
• Test seams: internal WithHistoryItem(commandId, now) / GetCommandHistoryWeight(commandId, now) overloads let tests inject strictly-increasing timestamps. Public overloads pass DateTimeOffset.UtcNow; the public IRecentCommandsManager contract is unchanged, so MainListPage needed no changes.

Design constants

• Half-life: 3 days
• Base weight: 10, frequency weight: 10, max weight: 70
• Store cap: 500
• Legacy backdate: 1 day

Tests

• Rewrote the position-bucket tests as explicit time-decay tests (recency decay, frequency weighting, cap, legacy migration, serialization round-trip).
• Kept the tier-invariant tests unchanged in spirit (tier still dominates; frecency only reorders within a tier).
• Build: x64 Debug, exit 0.
• Microsoft.CmdPal.UI.ViewModels.UnitTests: 150/150 pass (RecentCommandsTests 14/14).

No emdashes. Change is atomic to this phase and confined to CommandPalette.slnf.