fix(apiproxy): configure session export compression

Session-log ZIP entries always used DEFLATE level 6 even though compression level is a deployment tradeoff: CPU-constrained hosts may prefer low latency while bandwidth-constrained hosts may prefer smaller archives. A hardcoded level also violated the repository rule that deployment-varying plugin choices live in validated Config.

Add sessionExportCompressionLevel to ApiProxyService.Config as an integer 0-9 with default 6, resolve the same default once for direct createApiProxy callers, and pass the required level into the streaming module. Tests prove schema defaulting and rejection as well as a level-0 versus level-9 archive-size difference with identical extracted content. The generated config catalog, bilingual gateway README, and feature note document the knob and its tradeoff.

.agents/notes/implemented/feature/2026-08-10-web-session-log-export.md

@@ -10,7 +10,7 @@ The Trajectory view had no way to hand a debugging artifact to a human: the raw
 
 ## Decision
 
-- **The export is a host-only download, not an RPC**: `GET /api/session.export?sessionId=…&includeDescendants=true` streams one ZIP attachment. Every file is a session's **stored artifact text verbatim**: `readRaw` on the persistence service reads the backend's own durable bytes (the JSONL backend decodes its physical zstd frames, or returns plaintext) — never a reconstruction from parsed events, so packed-chunk rows, key order, and line breaks survive byte-for-byte — under its original base name (`session.jsonl` at the root, `subagents/<id>/session.jsonl` for descendants). Compression runs on the host with fflate's streaming `Zip`/`ZipDeflate` API, each entry deflated in bounded chunks as it is produced, so the response is chunked as it is generated and the host never holds the whole archive in one buffer (at most one descendant's artifact text beyond the preloaded root). At the 64 KiB response byte high-water mark, production waits for consumer pull to restore capacity; fflate's synchronous callback can add at most one bounded input push beyond that queue bound. No manifest is written — every file is byte-identical to the durable artifact and self-describing through its own header line.
+- **The export is a host-only download, not an RPC**: `GET /api/session.export?sessionId=…&includeDescendants=true` streams one ZIP attachment. Every file is a session's **stored artifact text verbatim**: `readRaw` on the persistence service reads the backend's own durable bytes (the JSONL backend decodes its physical zstd frames, or returns plaintext) — never a reconstruction from parsed events, so packed-chunk rows, key order, and line breaks survive byte-for-byte — under its original base name (`session.jsonl` at the root, `subagents/<id>/session.jsonl` for descendants). Compression runs on the host with fflate's streaming `Zip`/`ZipDeflate` API at validated `sessionExportCompressionLevel` 0–9 (default 6), letting deployments trade CPU and latency against archive size; each entry is deflated in bounded chunks as it is produced, so the response is chunked as it is generated and the host never holds the whole archive in one buffer (at most one descendant's artifact text beyond the preloaded root). At the 64 KiB response byte high-water mark, production waits for consumer pull to restore capacity; fflate's synchronous callback can add at most one bounded input push beyond that queue bound. No manifest is written — every file is byte-identical to the durable artifact and self-describing through its own header line.
 - **Error vocabulary is HTTP-native**: missing services → 500, a backend without per-session raw artifacts → 501, missing root session → 404 (all decided before any byte streams), and a descendant without a stored artifact → the stream errors (fail-loud, never silent under-export). Request abort remains cancellation instead of being rewritten as 500; request and response-consumer cancellation converge on the producer signal, which reaches lineage and persistence reads and terminates the active compressor. The carrier (`toFetchHandler`) already applies the `/api` trust fence; the GET branch sits beside the existing SSE GET routes, and `ApiProxy.downloads.sessionLog` (host-only, no wire envelope, absent from `IApiClient`) implements it.
 - **The UI just downloads**: the 导出 button hands the endpoint directly to the browser's native download manager, so JavaScript neither fetches nor buffers the ZIP; the `session.log` RPC that an earlier iteration shipped was removed — the download endpoint is its only consumer, and the repo rule is no public interface without a current owner. The client bundle carries no archive implementation.
 - The 导出 button lives in the Trajectory toolbar; the plugin exposes `exportLog` through the view's inject face (components never touch ctx) and resolves the view tab label through the locale service (`轨迹` in Chinese, `Trajectory` in English). In-flight state disables the button during the handoff; a synchronous browser-handoff failure surfaces in a visible alert bar, while HTTP delivery is owned and reported by the browser.