Refactor library architecture

[LesterEvSe]

Refactor the standard library and test suite

Resolves #18 for current state.

Reorganises the standard library into three conceptual parts and rewrites the
Rust test suite to eliminate duplicated boilerplate.

1. simf/lib/ — the library itself

The standard-library functions. Unchanged in this PR; only the surrounding
scaffolding (parts 2 and 3) is touched.

2. simf/ — main-function contracts

The test entry-point contracts, one main per area. Cleaned up with a clearer
structure and a shared helper.

• Renamed *_mock.simf → *_test.simf: these are test entry points, not mocks.
• Removed the IF_TEST_OVERFLOW witness flag. The overflow vs. fitting case
  is now encoded in the data: a checked_* result is compared against an
  expected: Option<T> witness, where None means overflow and Some(x) the
  fitting result. This drops one match level and one witness field from every
  contract.

3. tests/ — the Rust test suite

Rewritten. Previously, fund_script / spend_script and the spend-and-assert
logic were duplicated in every file, and u8_test.rs through u64_test.rs were
near-identical, differing only by type.

Shared spend plumbing — tests/common/mod.rs

Used by every test:

• fund — send sats to a program's script so it has a UTXO to spend.
• spend — build the transaction with a witness and broadcast it.
• run — fund + spend + assert in one call.
• Expect — the expected outcome of a spend.

uint test framework — tests/common/uint.rs

Covers operations uniform across every unsigned width (safe_add_N,
checked_mul_N, etc.). Built on a TestUint trait: each operation's logic is
written once as a generic function and instantiated per width. The only
per-width code is a short impl TestUint block.

Type-specific operations

Operations that exist for only one width are not forced into the common
interface. Dispatch indices leave room for them:

• 0..=191 — reserved for common operations.
• CUSTOM_BASE = 192 — start of type-specific operations.

A future u64_widen(...) -> u256, for example, adds a match arm at
fn_idx == 192 in u64_test.rs without touching the shared interface.

CUSTOM_BASE can be raised or lowered if 192 slots turn out to be the wrong
number.

[Hrom131]

LGTM!