feat: define where a beacon event lives in the log table

[hughgrigg]

The beacon sends a GET to a path on the site's own domain and puts its payload in the query string, so its rows arrive in the CloudFront access log like any other request. They carry different information all the same, and nothing said where that definition lives. It stays in cs_uri_query, read at query time. A column of its own was never available, because LogTable builds its columns from the fields the delivery was configured with and CloudFront has no field carrying somebody else's payload, and a view or a second table over the same objects meets the same wall since no SerDe parses a query string. searches already reads a search term out of the same column this way. src/beacon-events.ts holds the envelope both halves read: three parameters, one letter each, built in the browser with beaconQueryString and read back as SQL with beaconEventColumn and friends, so one definition covers both directions. Partitions written before the beacon shipped answer no rows rather than nulls, because a beacon row is identified by the path it was sent to, which is the one shape of schema change an immutable store takes without argument. What each event type carries beside the envelope is left to the beacon's own issue, and beaconSchemaVersion rides on every row so that arriving later costs no reinterpretation of rows already written.

Resolves #100

• Conventional commit message, used as the title

• Conventional branch name, like feat/concise-description

• Full check with pnpm run check passed

• Rebased off latest main

• User-facing behaviour is documented in docs/

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📒 Files selected for processing (5)

• docs/log-table/README.md
• src/beacon-events.test.ts
• src/beacon-events.ts
• src/cdk/log-table.test.ts
• src/index.ts

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