Plug-ins: live effects of a document's first view share a 2,000 ms budget

While the app draws a document on screen, the live-effect plug-in runs of
its first view share a 2,000 ms budget (VectorCraft's own value); each run
takes the lesser of its 5,000 ms limit and what is left. A stage left out
for time, or one that times out, fails, has no plug-in or meets Safe Mode,
draws its input art unchanged and is deferred, and the status bar says how
many. Later drawings leave a deferred stage as it is until its input or
parameters change.

- effect.deferredStatus lists the deferred stages with object, effect
  index, fill or stroke, plug-in and reason, objects in symbol, pattern,
  brush and opacity-mask art included.
- Effect › Refresh Deferred Effects (effect.refreshDeferred {ids?}) runs
  them again with a fresh budget and returns {refreshed, stillDeferred,
  failed}.
- Each document has its own budget and deferred stages; exports, Expand
  Appearance and commands run every stage under the per-call limit.
- The web build has no clock for the budget: its first view runs no
  plug-in effect, and a refresh runs them.

Docs: docs/mcp.md, docs/plugins.md, docs/target-app-parity.md.

docs/plugins.md

@@ -137,6 +137,19 @@ An export runs where the export runs: with Background Export on, on the export's
 plug-in between two of those 20 M-instruction slices; nothing is written then. An import stops the same way when the
 open it runs in is cancelled, and nothing opens then.
 
+### Live effects while a document opens
+
+While the app draws a document on screen, the live-effect runs of its first view share a 2,000 ms budget
+(`vectorcraft_plugins::deferred::OPENING_BUDGET_MS`, VectorCraft's own value), measured from the first run. Each run
+takes the lesser of its 5 s limit and what is left of the budget; once the budget is spent no run starts, and a result
+that comes back after it is not used. A stage left out this way, or one that times out, traps, returns output the host
+refuses or has no plug-in, draws its input art unchanged and is deferred. The status bar then says how many stages
+are deferred, and later drawings leave them as they are until their input or parameters change. Effect › Refresh
+Deferred Effects (`effect.refreshDeferred`) runs them again with a fresh budget, and `effect.deferredStatus` lists
+them with the reason (`budget`, `timeout`, `missing`, `failed`, `safeMode`). Each document has its own budget.
+Exports, Expand Appearance and commands run every stage under the 5 s limit. The web build has no clock for the
+budget, so there the first view runs no live effect and Refresh Deferred Effects runs them.
+
 ### Heavy filters
 
 Some filters do heavy but legitimate work, such as routing a plotter's pen, packing shapes onto a sheet or running a