fix(platform-web): broadcast broadcasted writes and resets while the signal is not observed

[dangreen]

Why

BroadcastChannelStorage opened its channel only in sub, and syncedStored subscribes to the storage when the signal gets its first subscriber. set posted through this.#channel?.postMessage(...), so a write from a tab that did not observe the signal itself was silently dropped. del was a no-op, so $signal(undefined) never reached other tabs either. Probe in the browser runner, tab A writes and tab B listens:

no subscriber in writer tab:   target = "idle"     <- 'logout' lost
with subscriber in writer tab: target = "logout"
after $a(undefined):           target = "logout"   <- reset lost

The docs example, a logout button calling $authEvent('logout'), is exactly the first case: the tab with the button has no reason to observe $authEvent. The existing spec masked it by attaching an effect to the source signal before writing.

What

• #open(key) creates the channel lazily with ??= and holds the single typeof BroadcastChannel check. set and sub share it; unsubscribing still closes and clears the channel, and the next write opens a fresh one.
• del posts null, which the subscription handler already maps to the default value.
• Tests: should post messages while the signal is not observed and should broadcast a reset.

Size

The lazy channel and the reset cost bytes in the only bundle of the package. The limit is re-pinned on the 0.05 kB step:

bundle	before	after	limit
All publics (Gzip)	3399 B	3416 B	3.4 kB → 3.45 kB
All publics (Brotli)	3027 B	3039 B	3.05 kB

Checks

• oxlint, tsc --noEmit, vitest run (60 tests, browser mode) and size-limit in packages/platform-web pass.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.29%. Comparing base (82d45cf) to head (ab8db7f).

Files with missing lines	Patch %	Lines
packages/platform-web/src/broadcast.ts	60.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #247      +/-   ##
==========================================
- Coverage   83.32%   83.29%   -0.03%     
==========================================
  Files          98       98              
  Lines        2554     2556       +2     
  Branches      550      551       +1     
==========================================
+ Hits         2128     2129       +1     
  Misses        314      314              
- Partials      112      113       +1     

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