feat: exclusion-aware frame peek for filter-sensitive consumers

[divanshu-go]

What

Adds a peek API that makes the persistent stream's latched frame safely consumable by filter-sensitive callers (screenpipe's visual-change checks), and fixes a filter-update staleness bug that also affects capture().

before (per visual check, ~every 3-5s per monitor):
  capture_monitor_sync → spawn_blocking → manager lock → maybe create stream
                       → maybe update filter → clone latch under global lock

after:
  Monitor::peek_image_scaled_excluding → manager lock (validate only)
                                       → clone latch OUTSIDE the lock
  (falls back to capture when no stream / filter or resolution mismatch)

Peek API

• peek_latest_frame_matching(monitor_id, width, height, ids) — returns the latch only when the running stream matches what a capture call with those parameters would use: same resolution (capture recreates on any width/height change) and same exclusion filter (the sorted comparison capture() uses). On mismatch the caller falls back to a capture, which installs the requested parameters. A returned frame is therefore always interchangeable with a captured one — no stream creation, no filter churn, no blocking-thread hop, same ≤1-frame-interval latch staleness.
• Monitor::peek_image_excluding / peek_image_scaled_excluding compute target dims with the same scaled_dims logic as the corresponding capture_image* methods, so callers can't drift.
• Per-stream filter epoch: the matching peek validates under the manager lock, clones the frame after releasing it, and rejects if the epoch moved — a concurrent capture swapping the filter mid-read can't hand back pixels the validation never saw.

Filter-update correctness (pre-existing bug, also hardens capture())

update_exclusions() installed the new ContentFilter but left the latched frame — composited under the old filter — in place, so for up to one frame interval both capture() and peeks served pixels the new exclusion list claims are filtered out (e.g. a freshly-ignored password manager). The latch is now dropped after a successful filter install; consumers wait ≤1 frame interval for the first post-update delivery.

Lock hygiene

All latch reads (peeks and capture()'s fast paths) clone the multi-MB frame buffer outside the global manager lock — a copy on one monitor no longer stalls captures, filter updates, or peeks on other monitors.