[Segment Cache] Predictable fallback param encoding

[acdlite]

When a segment includes a fallback parameter, during prerendering, a random placeholder is used to generate the response. We then use this response to generate a key path for each segment. We need this path to be predictable so the server can create a rewrite for it; it can't include random placeholders.

So, this updates the segment generation to replace the placeholder values with a "template" string, e.g. [paramName].

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Tests Passed

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Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	acdlite/next.js segment-cache-predictable-fallback-param-encoding	Change
buildDuration	17.8s	16.2s	N/A
buildDurationCached	15.3s	12.9s	N/A
nodeModulesSize	418 MB	418 MB	⚠️ +24.3 kB
nextStartRea..uration (ms)	429ms	428ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	acdlite/next.js segment-cache-predictable-fallback-param-encoding	Change
5306-HASH.js gzip	54 kB	53.9 kB	N/A
8276.HASH.js gzip	169 B	168 B	N/A
8377-HASH.js gzip	5.46 kB	5.46 kB	N/A
bccd1874-HASH.js gzip	52.9 kB	52.9 kB	✓
framework-HASH.js gzip	57.5 kB	57.5 kB	N/A
main-app-HASH.js gzip	240 B	242 B	N/A
main-HASH.js gzip	34.4 kB	34.4 kB	N/A
webpack-HASH.js gzip	1.71 kB	1.71 kB	N/A
Overall change	52.9 kB	52.9 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	acdlite/next.js segment-cache-predictable-fallback-param-encoding	Change
polyfills-HASH.js gzip	39.4 kB	39.4 kB	✓
Overall change	39.4 kB	39.4 kB	✓

Client Pages

	vercel/next.js canary	acdlite/next.js segment-cache-predictable-fallback-param-encoding	Change
_app-HASH.js gzip	193 B	193 B	✓
_error-HASH.js gzip	193 B	193 B	✓
amp-HASH.js gzip	512 B	510 B	N/A
css-HASH.js gzip	343 B	342 B	N/A
dynamic-HASH.js gzip	1.84 kB	1.84 kB	✓
edge-ssr-HASH.js gzip	265 B	265 B	✓
head-HASH.js gzip	363 B	362 B	N/A
hooks-HASH.js gzip	393 B	392 B	N/A
image-HASH.js gzip	4.59 kB	4.58 kB	N/A
index-HASH.js gzip	268 B	268 B	✓
link-HASH.js gzip	2.35 kB	2.35 kB	N/A
routerDirect..HASH.js gzip	328 B	328 B	✓
script-HASH.js gzip	397 B	397 B	✓
withRouter-HASH.js gzip	323 B	326 B	N/A
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	3.59 kB	3.59 kB	✓

Client Build Manifests

	vercel/next.js canary	acdlite/next.js segment-cache-predictable-fallback-param-encoding	Change
_buildManifest.js gzip	748 B	747 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	acdlite/next.js segment-cache-predictable-fallback-param-encoding	Change
index.html gzip	525 B	524 B	N/A
link.html gzip	539 B	538 B	N/A
withRouter.html gzip	521 B	521 B	✓
Overall change	521 B	521 B	✓

Edge SSR bundle Size Overall increase ⚠️

	vercel/next.js canary	acdlite/next.js segment-cache-predictable-fallback-param-encoding	Change
edge-ssr.js gzip	129 kB	129 kB	N/A
page.js gzip	208 kB	208 kB	⚠️ +124 B
Overall change	208 kB	208 kB	⚠️ +124 B

Middleware size

	vercel/next.js canary	acdlite/next.js segment-cache-predictable-fallback-param-encoding	Change
middleware-b..fest.js gzip	671 B	666 B	N/A
middleware-r..fest.js gzip	155 B	156 B	N/A
middleware.js gzip	31.3 kB	31.3 kB	N/A
edge-runtime..pack.js gzip	844 B	844 B	✓
Overall change	844 B	844 B	✓

Next Runtimes

	vercel/next.js canary	acdlite/next.js segment-cache-predictable-fallback-param-encoding	Change
274-experime...dev.js gzip	322 B	322 B	✓
274.runtime.dev.js gzip	314 B	314 B	✓
app-page-exp...dev.js gzip	375 kB	375 kB	N/A
app-page-exp..prod.js gzip	130 kB	130 kB	N/A
app-page-tur..prod.js gzip	143 kB	143 kB	N/A
app-page-tur..prod.js gzip	139 kB	139 kB	N/A
app-page.run...dev.js gzip	363 kB	363 kB	N/A
app-page.run..prod.js gzip	126 kB	126 kB	N/A
app-route-ex...dev.js gzip	37.6 kB	37.6 kB	✓
app-route-ex..prod.js gzip	25.6 kB	25.6 kB	✓
app-route-tu..prod.js gzip	25.6 kB	25.6 kB	✓
app-route-tu..prod.js gzip	25.4 kB	25.4 kB	✓
app-route.ru...dev.js gzip	39.2 kB	39.2 kB	✓
app-route.ru..prod.js gzip	25.4 kB	25.4 kB	✓
pages-api-tu..prod.js gzip	9.69 kB	9.69 kB	✓
pages-api.ru...dev.js gzip	11.6 kB	11.6 kB	✓
pages-api.ru..prod.js gzip	9.68 kB	9.68 kB	✓
pages-turbo...prod.js gzip	21.9 kB	21.9 kB	✓
pages.runtim...dev.js gzip	27.7 kB	27.7 kB	✓
pages.runtim..prod.js gzip	21.9 kB	21.9 kB	✓
server.runti..prod.js gzip	916 kB	916 kB	✓
Overall change	1.2 MB	1.2 MB	✓

build cache Overall increase ⚠️

	vercel/next.js canary	acdlite/next.js segment-cache-predictable-fallback-param-encoding	Change
0.pack gzip	2.1 MB	2.1 MB	⚠️ +1.19 kB
index.pack gzip	75.8 kB	75.6 kB	N/A
Overall change	2.1 MB	2.1 MB	⚠️ +1.19 kB

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Commit: 59c26e6