docs: align README shielded examples with current compiler behavior

[Osraka]

The main README currently contains two behavior mismatches with the compiler on the current seismic branch (e0eb91ec8).

First, the ConfidentialWallet usage example does not compile with:

ssolc --evm-version mercury --bin ConfidentialWallet.sol

The compiler reports:

• a public function returning suint256;
• an address == saddress comparison;
• contract instantiation from an saddress;
• a call to the private addFunds function through another contract instance.

Second, the mappings section says that shielded types are supported for keys and/or values. The compiler rejects a shielded key with Shielded types are not allowed as mapping keys, while a public key with a shielded value compiles. The README's error-code table already describes shielded mapping keys as disallowed.

This is more than a wording cleanup: the primary example teaches APIs that current users cannot compile. The repository already has a test/libsolidity/syntaxTests/docExamples/ pattern for keeping documentation examples checked by the compiler.

Would the preferred scope be a single PR that:

1. replaces the wallet example with a currently supported, compilable example;
2. documents that mapping values may be shielded but keys may not; and
3. adds a focused syntax test for the README example to prevent future drift?

I can prepare that as a documentation-and-test-only change, without touching compiler behavior.

[huklaa]

I'd like to work on this. I have a documentation-and-test-only fix in progress that:

• replaces the non-compiling shielded wallet example with a currently supported example;
• clarifies that shielded mapping values are supported but shielded mapping keys are not; and
• adds a focused syntax test so the README example stays compiler-checked.

I'll keep the scope limited to docs + regression coverage and won't change compiler behavior.