perf: avoid O(n²) name scanning in scope generate/unique (#17844)

## Summary

`Scope.generate()` and `ScopeRoot.unique()` search for available names
by iterating from suffix `_1` upward. When the same preferred name is
generated many times (e.g. `text` is generated 482 times in a large
component), the Nth call re-scans all N-1 already-taken names — O(n²)
total work.

This adds a `#name_counters` Map to `ScopeRoot` that tracks the next
suffix to try per name, so each call resumes from where the last one
left off. Generated names are identical to before.

## Benchmark (interleaved, best-of-3 rounds)

| Component | Min | Median |
|---|---|---|
| Realistic (~80 lines) | ~1% | ~7% |
| Medium (316 lines) | ~1% | ~5% |
| Large (642 lines) | ~7% | ~2% |
| XLarge (1302 lines) | **~11%** | **~10%** |

## Test plan

- [x] All snapshot tests pass (name generation unchanged)
- [x] All validator, compiler-error, runtime-runes, runtime-legacy tests
pass

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.changeset/smooth-pens-jump.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+perf: avoid O(n²) name scanning in scope `generate` and `unique`

packages/svelte/src/compiler/phases/scope.js

@@ -713,8 +713,18 @@ export class Scope {
 		}
 
 		preferred_name = preferred_name.replace(/[^a-zA-Z0-9_$]/g, '_').replace(/^[0-9]/, '_');
-		let name = preferred_name;
-		let n = 1;
+
+		// Use cached counter to skip names already known to be taken (avoids O(n²) scanning)
+		let n = this.root.next_counter(preferred_name);
+		let name;
+
+		if (n === 0) {
+			name = preferred_name;
+			n = 1;
+		} else {
+			name = `${preferred_name}_${n}`;
+			n++;
+		}
 
 		while (
 			this.references.has(name) ||
@@ -725,6 +735,7 @@ export class Scope {
 			name = `${preferred_name}_${n++}`;
 		}
 
+		this.root.set_counter(preferred_name, n);
 		this.references.set(name, []);
 		this.root.conflicts.add(name);
 		return name;
@@ -852,18 +863,49 @@ export class ScopeRoot {
 	/** @type {Set<string>} */
 	conflicts = new Set();
 
+	/**
+	 * Tracks the next suffix counter per name to avoid O(n) rescanning in generate/unique.
+	 * @type {Map<string, number>}
+	 */
+	#name_counters = new Map();
+
+	/**
+	 * @param {string} name
+	 * @returns {number}
+	 */
+	next_counter(name) {
+		return this.#name_counters.get(name) ?? 0;
+	}
+
+	/**
+	 * @param {string} name
+	 * @param {number} value
+	 */
+	set_counter(name, value) {
+		this.#name_counters.set(name, value);
+	}
+
 	/**
 	 * @param {string} preferred_name
 	 */
 	unique(preferred_name) {
 		preferred_name = preferred_name.replace(/[^a-zA-Z0-9_$]/g, '_');
-		let final_name = preferred_name;
-		let n = 1;
+		let n = this.#name_counters.get(preferred_name) ?? 0;
+		let final_name;
+
+		if (n === 0) {
+			final_name = preferred_name;
+			n = 1;
+		} else {
+			final_name = `${preferred_name}_${n}`;
+			n++;
+		}
 
 		while (this.conflicts.has(final_name)) {
 			final_name = `${preferred_name}_${n++}`;
 		}
 
+		this.#name_counters.set(preferred_name, n);
 		this.conflicts.add(final_name);
 		const id = b.id(final_name);
 		return id;