SDK-32: Add shared audience SDK core package

[nattb8]

Summary

New @imtbl/audience-core package that provides the shared foundation both the web SDK and pixel build on. This lets web SDK and pixel development happen in parallel.

What's in the core:

• Cookie-based anonymous ID (imtbl_anon_id) -- shared between pixel and SDK for identity stitching across surfaces
• HTTP transport -- POST to /v1/audience/messages with publishable key auth, keepalive: true for page unload reliability
• Batched queue -- flushes at 20 messages or 5s interval, persists to localStorage so events survive navigation
• Context collection -- device fingerprint signals (userAgent, locale, timezone, screen, page info) and UTM param extraction
• Types -- message types matching the audience service API contract (track, page, screen, identify, alias)

Key design decisions:

• Anonymous ID uses a cookie (not localStorage) so it works across the pixel script tag and npm SDK on the same domain
• Context collection is parameterised by library name so the pixel and SDK identify themselves differently
• collectPageProperties() and collectUtmParams() are exported separately so the pixel can use them for auto-capture and the SDK can use them in page()
• No external dependencies

Test plan

• 27 unit tests passing (cookie, queue, transport, context, storage)
• TypeScript strict mode passes
• CI lint/build

Linear: https://linear.app/imtbl/issue/SDK-32

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