fix(web): break effect_update_depth_exceeded in HomePage health-poll diff

[explosivebit]

Summary

Hotfix for runtime error reported by user after F12 merge:

Uncaught Svelte error: effect_update_depth_exceeded
Maximum update depth exceeded. This typically indicates that an effect
reads and writes the same piece of state.

Root cause

HomePage.svelte:130 $effect for breach detection:

• reads prevHealthSnapshot via detectBreaches(prevHealthSnapshot, next, ...)
• writes prevHealthSnapshot = next at the bottom

Both on the same $state(...) signal → Svelte 5 (correctly) detects the cycle and abort-loops the effect.

Fix

prevHealthSnapshot is a "previous tick" memo — no template reads it, no other effect depends on it. Reactivity is unnecessary; the value lives across re-runs via the component's closure scope. Demoted to plain let.

Verify

• npx svelte-check — 0/0/436.
• npm test — 68/68 (no test impact; pure-function notify lib unchanged).
• Manual: dev server loads, browser console shows 0 errors after navigation.

Why $state was wrong here

In Svelte 5 runes, $state creates a reactive signal. Any effect that reads + writes the same signal triggers a cycle. For "memoized previous value" between successive runs of one effect, a plain let survives in closure scope without becoming reactive — that's the idiomatic pattern.

Refs: PRD-007 RFC-006

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