Backport of #78066 to 15.3.x

---------

Co-authored-by: Sebastian "Sebbie" Silbermann <sebastian.silbermann@vercel.com>

Backport of #78149 to 15.3.x

### What?

Backport #75605

### Why?

x-ref: https://vercel.slack.com/archives/C06DGJ420TZ/p1744644426250019

Backport of #78325 to 15.3.x

Backport of #77963 to 15.3.x

Co-authored-by: Jiachi Liu <inbox@huozhi.im>

#78453 is a bugfix for sourcemap on windows

Backport of #78874 to 15.3.x

Co-authored-by: Donny/강동윤 <kdy1997.dev@gmail.com>

Backport of #78261 to 15.3.x

Co-authored-by: Will Binns-Smith <wbinnssmith@gmail.com>

…:LocalOutput (#78488) (#78883)

This is a straightforward backport of
#78488. There were only a few
minor merge conficts.

This *mostly* impacted use of Turbopack with persistent caching (which
is canary-gated), but we saw some reports of it without persistent
caching, so this backports the fix.

Tested with

```
cargo check
cargo test -p turbo-tasks-backend
cargo test -p turbo-tasks-memory
```

…79362)

Backport of #79309 to 15.3.x

…79526)

Without this, some files may fail to open correctly.

Internal discussion:
https://vercel.slack.com/archives/C07UWUAF95Z/p1747261772629689

### What?

Fix the usefulness detector for the React Compiler

### Why?

JSX was also a target of the React Compiler

#79558)

### What?

Add tests for the interestingness detection for the RC

)

Reverts #77797
Reinstate the `Vary` header to prevent browsers from incorrectly reusing
RSC responses during bfcache restores.

Since the corresponding change has been released in Vercel CLI, we can
re-enable the test.