refactor(assembly): split tests.rs into thematic test modules

[haileymarshall]

Closes #3065

Rationale

crates/assembly/src/tests.rs had grown to 7.6k lines / 233 tests, which makes it slow to
navigate, causes unrelated changes to collide in the same file, and buries which area a
failing test belongs to. #3065 asks for it to be split up now that #3073 has landed. Smaller
thematic modules make it obvious where a new test belongs and keep future diffs scoped to the
area they touch.

What changed

Splits tests.rs into crates/assembly/src/tests/:

• One module per existing // SECTION header (simple programs, libraries, constants,
  assertions, imports, errors, etc.). Test code is moved verbatim.
• The MAST TESTS section had grown into a ~3.7k-line catch-all, so it is further split by
  theme: mast, kernels (incl. syscall resolution), forest_merge, linking_imports,
  package_surface, link_diagnostics, cross_module_constants, link_cycles,
  import_regressions, symbol_resolution, link_expansion, num_locals,
  misc_regressions. Happy to regroup if you'd prefer different boundaries.
• Shared imports, helpers, and the assert_assembler_diagnostic! macro live in tests/mod.rs;
  each module starts with use super::*;.
• Insta snapshot files are renamed to their new module paths and their source: headers
  updated. The orphaned nested_blocks_hash.snap is deleted (its assertion was removed
  in Simplify MastForestBuilder with builder-local refs #3142).
• Added a CHANGELOG.md entry.

Test plan / how to review

• Test code is moved verbatim — no test logic was changed. Reviewing module-by-module against
  the old // SECTION boundaries should show only relocation plus the use super::*; preamble.
• Test count is unchanged: cargo test -p miden-assembly runs the same 233 tests as before
  the split (plus 1 in the pre-existing package.rs), all passing.
• Snapshot files are git mv-renamed, so the diff shows them as renames; only their source:
  headers changed.
• cargo fmt (stable and nightly) and
  cargo clippy -p miden-assembly --all-targets --all-features are green.

[huitseeker]

So sorry about the late review. This LGTM, but needs a rebase: since the PR branched, next added four tests, removed one, and changed two function bodies. Those changes must be deliberately moved into the new modules during a refresh.