build(hax): extract almost-upstream libcrux (mod hax v0.3.7)

[cfm]

Fixes #258, after a bit of run-around: see #238 (comment).

• Depends on soundness: split F* caches, then drop or admit contracts that have been failing silently all along #278

Following up on #222 (comment) per cryspen/libcrux#1397 (comment).

[cfm]

Incremental progress:

• We still need a fork of cryspen/libcrux, but only until there's a published crate that uses last week's hax and hax-lib versions 0.3.7.
  • This required updating rand_core to v0.10, superseding build(deps): bump rand_core from 0.9.3 to 0.10.0 in /securedrop-protocol #264 and build(deps): bump rand_chacha from 0.9.0 to 0.10.0 in /securedrop-protocol #267 towards Update rand_core and rand_chacha to 0.10.x #258.
• We no longer need a fork of cryspen/hpke-rs, since we can patch it at the workspace level to use hax-lib version 0.3.7. But we do need to install it from Git rather than crates.io until there's a published crate that uses libcrux-sha3 version 0.0.9.

This seems to resolve the extraction blockers highlighted in #222 (comment) and cryspen/libcrux#1397 (comment). There are still verification blockers I'm looking into.

[cfm]

Tomorrow I'll need to reconcile this with #260.

[cfm]

The remaining verification failures here are the ones newly surfaced by #271:

• Error: (19) * Error 19 at Securedrop_protocol_minimal.Primitives.Xwing.fst(218,2-257,85): and two more can be fixed via key-generation functions can probably be simplified after the crypto-provider refactoring #273.
• Error: (19) * Error 19 at Securedrop_protocol_minimal.Primitives.Pad.fst(15,62-15,71): just needs a precondition so it's panic-free.

[cfm]

I'll need to rebase and probably rework this now that it's based on #278 rather than #272.

[cfm]

I've finally got this extracting and verifying cleanly after #278, without any of the extra F* models I'd added in e2c9471. My fork cfm/libcrux@hax-lib-0.3.7 is now just a single commit on top of upstream for compatibility with hax v0.3.7. Once cryspen/libcrux#1478 is merged, we can pin to their main instead until new crates are published.