refactor(router): compare tail and prefix segments in place (#199)

<!-- claude-projects-attribution: {"github_login":"dinwwwh"} -->
_Requested by **dinwwwh** · [project
thread](https://claude.ai/code/project/chan_01UcKMz91tMarPV1kWLVgLYE?thread=cmsg_01UcKMz91tMarPV1kWLVgLYE6swgSUM9SyULmBVdRjY2t7)_

## What

`matchesSegment` was a one-line function used in two places:
`matchesCatchAll`, for the segments after a catch-all param, and the
closure that `createPrefixMatcher` returns. Both now do the comparison
in place, with a comment that `undefined` stands for a param, which
needs a non-empty segment.

## Why

Each top-level function costs V8 about 15k instructions to set up at
import, and about as much again to compile when it is first called (see
#198). This removes one function from every import. It also removes one
compile from the first request that reaches a catch-all, or a prefix
with a param.

## Measurements

Instruction counts from callgrind, `node --single-threaded`, averaged
over hash seeds 1 and 2:

| | before | after |
|---|---|---|
| min / min+gzip | 5043 / 2100 B | 5053 / 2098 B |
| import | 2.167M | 2.164M |
| REST cold+10 (includes a catch-all request) | 6.041M | 6.017M (−0.4%)
|
| steady, catch-all / tail | 4985 / 5256 | 4979 / 5249 |
| steady, prefix matcher with a param / with `%` | 2453 / 4082 | 2422 /
4052 (−1%) |

Cold+3 on the REST set reaches no catch-all, so it changes only by the
import saving.

## Checks

- Tests pass with 100% coverage.
- The model fuzzers, including the late-add and prefix ones, find 0
diffs.
- Stryker finds 68 survivors, the same as on main. None are on the new
comparisons.

---
_Generated by [Claude
Code](https://claude.ai/code/session_01VVaw7Sutbw6xMTMyDFeNfw)_

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

Author: dinwwwh

packages/router/src/index.ts

@@ -323,7 +323,9 @@ export function createPrefixMatcher(prefix: `/${string}`): (pathname: `/${string
         end = pathname.length
       }
 
-      if (!matchesSegment(expected, decode(pathname.slice(start, end)))) {
+      // `undefined` stands for a param, which needs a non-empty segment.
+      const segment = decode(pathname.slice(start, end))
+      if (expected === undefined ? segment === '' : segment !== expected) {
         return false
       }
 
@@ -627,18 +629,17 @@ function matchesCatchAll(
   }
 
   for (let i = 0; i < tail.length; i++) {
-    if (!matchesSegment(tail[i], segments[end + i]!)) {
+    const expected = tail[i]
+    const segment = segments[end + i]!
+    // `undefined` stands for a param, which needs a non-empty segment.
+    if (expected === undefined ? segment === '' : segment !== expected) {
       return false
     }
   }
 
   return true
 }
 
-function matchesSegment(expected: string | undefined, segment: string): boolean {
-  return expected === undefined ? segment !== '' : segment === expected
-}
-
 // `params` is spread only for a param named `__proto__`, not on every pass.
 /* oxlint-disable oxc/no-accumulating-spread */
 function toParams<T>(route: Route<T>, segments: readonly string[]): Record<string, string> {