Part of https://github.com/zed-industries/zed/issues/57349

`MarkdownElement` re-highlights every code block with tree-sitter on
every frame.
Each visible code block pays a full parse per frame, which makes agent
panel streaming and hover popovers janky once the content grows.

This PR computes code block highlights once, in the background parse
task, instead of at render time:

- Highlights are resolved into `(Range<usize>, HighlightId)` runs at
parse time; each entry stores the grammar and `HighlightMap` it was
resolved against and is served only while the grammar still holds the
pointer-identical map (`ResolvedHighlights::is_current`), so theme
changes need no invalidation hooks, and a block whose entry went stale
falls back to direct fragment highlighting at render until the next
parse.
- `Language::highlight_text_resolved` memoizes results in a
content-keyed LRU (4MB budget, 512KB per-entry cap), so the re-parses
triggered by streaming appends reuse the highlights of unchanged blocks
instead of re-running tree-sitter on them.
- Blocks larger than the per-entry cap bypass the cache and are
highlighted directly, as before.

`CostBudgetedLru` is generic rather than specialized because a follow-up
PR reuses it for per-buffer highlight caching in
`BufferSnapshot::chunks`; the capture flattening keyed by grammar index
(`HighlightCaptureRef`) is shared with that follow-up as well.

## Benchmarks

`cargo bench -p benchmarks --bench markdown_renderer` renders generated
documents that mix paragraphs, headings, tables, and Rust code blocks.

**Run 1 — baseline, `main` (`d6449a9e3f`):**

```
Markdown render/min_bytes/5000    time: [1.2956 ms 1.2974 ms 1.3016 ms]
Markdown render/min_bytes/10000   time: [1.7919 ms 1.7955 ms 1.8026 ms]
Markdown render/min_bytes/50000   time: [6.5510 ms 6.6241 ms 6.6671 ms]
Markdown render/min_bytes/250000  time: [37.644 µs 2.4819 ms 4.3791 ms]   <- bimodal already on main
```

**Run 2 — this branch:**

```
Markdown render/min_bytes/5000    time: [940.11 µs 945.28 µs 948.79 µs]
                                  change: [-29.317% -28.385% -27.607%] (p = 0.00 < 0.05)
Markdown render/min_bytes/10000   time: [1.1162 ms 1.1331 ms 1.1407 ms]
                                  change: [-40.859% -39.151% -37.719%] (p = 0.00 < 0.05)
Markdown render/min_bytes/50000   time: [2.4943 ms 2.9225 ms 3.1051 ms]
                                  change: [-80.088% -67.885% -58.058%] (p = 0.00 < 0.05)
Markdown render/min_bytes/250000  time: [43.442 µs 44.661 µs 45.514 µs]
                                  change: [-98.210% -95.806% +5.5167%] (p = 0.17 > 0.05)
```

Over the same runs, GPUI reports window draw mean 2.22 ms → 1.26 ms, p99
31.2 ms → 16.0 ms, and the share of frames that blow the frame budget
drops from 3.9% to 2.0%.

Note: at the 250,000-byte case root blocks render lazily, which makes
the measurement bimodal and inconclusive in both configurations.

Release Notes:

- Improved rendering performance of markdown code blocks (agent panel,
hover popovers, markdown preview)