Provide consistent structured logs across adapters #119

Description

Emit structured, level-configurable logs consistently on Fastly, Cloudflare, Spin and Axum.

Manifest logging levels/endpoints exist and Fastly/Axum initialize platform loggers. RequestLogger emits formatted tracing messages, while Cloudflare and Spin logger initialization returns success without installing a logger. This is partial logging support, not the requested cross-adapter structured output contract.

Steps to reproduce

1. Inspect manifest logging fields and each adapter initializer.
2. Compare the request logger events with a documented common field schema and output format.

Expected behavior

The same request facts and selected log level are observable through each adapter's supported output channel.

Impact

Telemetry format and visibility differ by runtime, making operations and log processing harder.

Proposed fix

Define shared structured fields and level behavior, bridge the existing log/tracing surfaces, and implement/document provider-specific sinks with application-owned redaction.

Done when

• A documented common event schema covers normal and failed requests.
• Configured levels suppress/emit the same logical events on each adapter.
• Cloudflare/Spin have an actual output path or an explicit supported integration.
• Runtime tests/examples demonstrate structured output without exposing secrets.

Related

• Parent: Complete portable request observability #118.
• Native correlated diagnostics: Axum: add safe production diagnostics and trusted-proxy handling #397.
• Logging destinations: Configure Fastly and Cloudflare logging destinations #123.
• Built-in request logger
• Manifest logging schema
• Cloudflare logger initialization
• Spin logger initialization
• Axum logging initialization
• Related PR feat(axum): add safe diagnostics and trusted proxy metadata #409; this PR alone does not establish completion of this issue.