fix(mapper): stop a slow control plane from piling up abandoned sessions

[trancong12102]

Third PR after the 2026-10-14 incident (#62 fixed the slow route reduction that triggered it). This one removes the amplifier.

What happened. Once map generation fell behind, every long poll waited in Batcher.AddNode → MapResponseFromChange. That wait had no context and no timeout. The client's watchdog cancelled after 120 s and it opened a new poll. The old session's connection was already registered, so it stayed counted. Production showed:

• 60–85 active_connections per node in /debug/batcher;
• 450 goroutines blocked in queueWork and 176 in MapResponseFromChange, the oldest for 180 minutes;
• the single worker building full maps for dead sessions ahead of live ones.

Change.

• AddNodeContext(ctx, …) is what serveLongPoll now uses; AddNode wraps it with context.Background(), so the 57 test callers are untouched. queueWork and the wait for the result both honour the context.
• When the context ends, the connection is detached, the same as the existing error path.
• Synchronous work carries the caller's context. A worker that reaches it after the caller gave up drops it unbuilt and counts it in slopscale_mapper_changes_dropped_total{reason="abandoned"}.
• The initial map waits at most 60 s (ErrBatcherBusy). The poll then answers 503 with Retry-After 30–89 s. Clients at capability version 147+ (Tailscale 1.104) honour that and wait instead of retrying at once; older ones back off as on any failure. A client that already left gets nothing written and a debug log.

Tests.

• TestAddNodeContextAbandoned runs on an unstarted batcher, so the initial map can never be built, like a saturated worker. It checks that the call returns with the cause, the connection stops counting, and once a worker starts the abandoned map is never built (the mapper is nil, so building it would panic). 200 runs pass under -race.
• go test -short ./hscontrol/ ./hscontrol/{mapper,state,servertest}/... and golangci-lint pass.

There is no automated test for the 503 path: initialMapWait is a 60 s constant. The switch in poll.go is three lines.

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