Fix broken relative links in README files

[UfukNode]

Some README links point at paths that no longer exist after crate/module moves.

Examples from the current next branch:

• core/README.md links to ./src/program.rs, ./src/kernel.rs, and ./src/stack/*, but those items now live under core/src/program/.
• miden-vm/README.md links to ../assembly, but the assembly crate is under crates/assembly.
• crates/lib/core/README.md links to ./docs/pcs/frie2f4.md and ./docs/stark/mod.md, while the existing files are ./docs/pcs/fri/frie2f4.md and ./docs/stark.md.
• The imported crate READMEs for serde-utils and crypto-derive link to license files one directory too shallow.

These are local relative links, so they are easy to miss in the current link checker because file:// targets are excluded.

[mehmetkr-31]

I'd like to take this one. Opened a draft PR.

Rather than fixing only the listed examples, I swept every non-http markdown link in the repo and resolved it against the filesystem: 15 broken before, 0 after. That turned up one you hadn't listed — crates/assembly/README.md links to ../processor, which is one level short now that assembly lives under crates/.

Five entries in the sweep are false positives worth knowing about if you ever want to automate this: three image files legitimately have parentheses in their names (DUP(n).png, MOVUP(n).png, MOVDN(n).png), which naive link regexes truncate, and two are angle-bracket autolinks rather than markdown links.