Keep the Editor responsive while a Forge write stalls

[sandersonstabo]

Under disk contention one Forge SQLite write can stall for ~44 s, and every navigation request queued behind it ("the Editor is still busy", "Images are still uploading").

• Forge answers up to 16 requests per connection concurrently. Subscribe/Unsubscribe keep accept order under the delivery driver, and write-gate stalls are logged.
• Transport: SessionRequester runs requests beside the session owner's, each on its own stream (up to 8 at once). Requesters share one correlation registry, and dropped requests are abandoned cleanly.
• Editor: draft saves, reads, uploads, and sends run on a mutation lane, so they no longer block navigation. Each draft scope runs one job at a time in queue order, so saves stay ordered and a send follows its save. Subscriptions and deliveries stay on the command loop. Losing the connection causes one reconnect, durable writes retry once under their stable identity, and shutdown drains the lane.

Draft saves were already coalesced app-side (DraftSync: one save in flight per scope, latest body wins).

Tests: new mutation_lane_tests::navigation_is_answered_while_a_draft_save_is_unanswered (a scripted Forge holds a save until a navigation request is answered). The frontend lib's 20 failures from master are unchanged.

🤖 Generated with Claude Code

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Summary by cubic

Keeps the Editor responsive when a Forge SQLite write stalls on a busy disk: navigation is answered while draft saves, uploads, and sends queue behind the write, instead of piling up behind "Editor is still busy".

• Forge answers up to 16 requests per connection concurrently; Subscribe/Unsubscribe stay in accept order under the delivery driver, and a write-gate wait or hold longer than two seconds is logged with the operation's name.
• Transport gains SessionRequester, which runs requests beside the session owner's on their own streams (up to 8 at once) against a shared correlation registry, with clean abandonment of dropped requests.
• The Editor's draft saves, reads, uploads, and sends run on a mutation lane, so they no longer block navigation; each draft scope runs one job at a time in queue order, keeping saves ordered and a send after its save. Subscriptions and deliveries stay on the command loop; a lost connection reconnects once, durable writes retry once, and shutdown drains the lane.
• Adds a scripted-Forge test that holds a save until a navigation request is answered; the frontend lib's 20 failures from master are unchanged.

^{Written for commit dca5f4f. Summary will update on new commits.}