fix(thumbnails): report per-frame read time and time left during Update Thumbnails

[thetalkingdrum]

Part of marcinz606#1232. A roll-scope Update Thumbnails on a slow SMB share took ~15 min for 49 ARWs at ~3% CPU. Each frame is one 15–20 s network read plus ~1 s of decode/render, and the status bar showed no timing, so the refresh looked hung.

Changes

• Worker (ThumbnailRenderWorker.process): times _decode_asset_preview_with_meta and run_pipeline separately with time.perf_counter(). progress is now (done, total, name, decode_s, render_s). A frame that fails reports the time the failed step took, so unreadable files on a slow share still count toward the estimate; a step that never ran reports 0.0.
• frame_started(index, total, name): emitted before each decode, so the first frame's read isn't silent.
• Controller: keeps a per-generation running mean (reset when refresh_thumbnails_for starts a generation; a pre-emption resume starts a new one). Formatting lives in the pure thumbnail_refresh_status(...):
  • time left (— ~N min left / — ~N s left) from done >= 2;
  • (reading source files, N s/frame) when mean decode > 3 s and > 2× mean render;
  • otherwise the existing text, unchanged.
  • frame_started uses the same function with in_flight=True: the frame being decoded counts as left, and only finished frames count as samples.
• Tooltip / User Guide: the fingerprint branch hasn't merged, so both setToolTip sites and docs/USER_GUIDE.md now say the roll refresh does every thumbnail, not "every stale" one.

Out of scope: stale-only refresh and render fingerprints (feat/thumbnail-fingerprint). Which frames the roll scope dispatches is unchanged.

Tests

• Worker: a scripted perf_counter checks the decode/render durations in the payload; frame_started fires before each decode; a failed decode and a failed render each report the time they took.
• thumbnail_refresh_status: no ETA at done=1 or on the last frame; seconds vs minutes; the read-bound hint only when decode dominates.
• Controller: the running mean resets on a new generation; frame_started shows the in-flight frame with time left.
• Updated the existing 3-arg progress connection.

make all: lint and type clean, 7327 passed.

🤖 Generated with Claude Code

https://claude.ai/code/session_01W1MEH5L17tDmpmz1H47dap

[github-actions]

Preview load metrics (github-linux-x64)

format	cold load	throughput
CR2	0.508 s	44.12 MPx/s
NEF	0.385 s	63.87 MPx/s
ARW	0.133 s	151.98 MPx/s
RAF	1.334 s	12.19 MPx/s
DNG	0.414 s	57.84 MPx/s

fixtures: rawsamples.ch · sha: 8ead9bc · full JSON