.ai import: symbol, pattern and brush definitions share the file's blend and envelope limits

Each definition's reader started with fresh blend layout and envelope
check budgets, so a file with many symbols, patterns or brush art
sections got those limits again for every definition. The reader of a
definition now takes the import's remaining budgets and hands back what
is left after reading, whether the definition is kept or not.

docs/target-app-parity.md

@@ -1214,6 +1214,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
     A `.ai` or EPS opened from its editing data keeps each artboard's center mark, cross hairs, video safe areas and video ruler pixel aspect ratio, and the file's Document Raster Effects Settings; the author comes from the header's `%%For` (the editing data's, else an EPS's own) when the file's File Info gives none, the header's custom colors that no swatch has come in as spot swatches, a named color the file defines takes the place of a default swatch of that name when the file has no palette, and an older file's `%%TemplateBox` places its artboard. A value of another type or out of range keeps its default (`crates/eps/src/import/ai/tests.rs`, `crates/eps/src/import/tests_native.rs`, `crates/engine/src/cmd/fileio/tests_aiimport.rs`).
     A `.ai` opened from its editing data opens its Objects on Path groups live, with their path, their objects in order along it, Pivot and Rotate, and their art where the file draws it; a group whose spacing has been dragged, or with options an Objects on Path group doesn't hold, opens as a plain group (`crates/eps/src/import/ai/tests.rs`).
     `.ai` symbol definitions and placed instances open as live symbols, in the Symbols panel's order; an instance of a symbol that isn't read, or whose appearance differs from its symbol's, comes in as the page draws it (`crates/eps/src/import/ai/tests.rs`).
+    Blends and envelopes in the definitions of a `.ai`'s symbols, patterns and brushes' art count against the same per-file limits on blend layout and envelope checks as the layers' art, so a file with many definitions does not get those limits again for each one (`crates/eps/src/import/ai/read/directives.rs`).
     A `.ai` or EPS whose editing data has brushes opens with the file's Calligraphic, Scatter, Art and Bristle brushes in the Brushes panel in place of the default ones, in the file's order: the Calligraphic size, roundness and angle with their variation, the Scatter size, spacing, scatter and rotation, the Bristle settings, and the Scatter and Art brushes' art. A brush library `.ai` (brushes and no layers) opens from its PDF part with its brushes, so Other Library lists them. Pattern brushes and brushes that can't be read are left out with a note; brushed paths still come in as their drawn look (part of #372 and #637; `crates/eps/src/import/ai/tests.rs`, `crates/eps/src/import/tests_native.rs`).
     In a `.ai` opened from its editing data, a path or type object with Offset Path, Outline Object or Outline Stroke on the whole object opens as itself with those effects live, in their order and shown or hidden; on a fill or stroke of type they stay on that fill or stroke. Offset Path keeps its Offset, Joins and Miter Limit, and Outline Stroke takes its width from the stroke. An Offset Path with a setting missing, out of range or not read keeps the drawn art, as does a path with a Path effect on one fill or stroke (`crates/eps/src/import/ai/style/tests.rs`, `crates/eps/src/import/ai/tests.rs`, `crates/eps/src/import/tests_native.rs`).
     In a `.ai` opened from its editing data, an object with a Tweak that moves nothing or Pucker & Bloat at 0 opens as itself with the effect live. A path, compound path or group whose only effect is Transform without copies, or a Free Distort whose corners make a parallelogram, opens as itself with it live, and so does an unstroked path or compound path with straight segments whose only effect is Twist. A gradient or pattern, an image, type, a clipping group, an opacity mask or effects inside the object, or a stroke with a Transform that scales, rotates or reflects, keep the drawn art (`crates/eps/src/import/ai/style/tests.rs`, `crates/eps/src/import/ai/tests.rs`, `crates/eps/src/import/ai/read/ops.rs`).