Trojan Source mitigation in the lexer is incomplete

## Description

Trojan Source (CVE-2021-42574 class) mitigation in the lexer is incomplete.

`liblangutil/Scanner.cpp` uses `validateBiDiMarkup()` to reject unbalanced bidirectional override markers in comments and string literals, but:
- NatSpec doc comments (`///` and `/** ... */`) are not validated, so unbalanced markers like `U+202E` (RLO) inside NatSpec are accepted.
- The validator does not include the isolate markers `U+2066..U+2069` (LRI/RLI/FSI/PDI), so unbalanced isolates are accepted even in regular comments/strings.

Impact: source can be rendered deceptively in BiDi-aware viewers (e.g., code visually appears inside a comment or tokens appear reordered) while the compiler compiles the real bytes, undermining human review and tooling assumptions.

## Environment

- Compiler version: `0.8.35-develop.2026.2.21+commit.30f20186` (local build from `develop`)
- Compilation pipeline (legacy, IR, EOF): legacy (default)
- Target EVM version (as per compiler settings): default
- Framework/IDE: N/A (CLI)
- EVM execution environment / backend / blockchain client: N/A (parsing stage)
- Operating system: macOS (Darwin)

## Steps to Reproduce

1. Create a file with an unbalanced `U+202E` inside a NatSpec doc comment:

```solidity
// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.0;

contract C {
function f() external {
bool isOwner = false;

/** audit: enforce owner check ‮*/ isOwner = true; // contains unbalanced U+202E

if (isOwner) { /* privileged path */ }
}
}
```

2. Compile (parsing is enough):

`solc --stop-after parsing .sol`

Expected: compiler rejects unbalanced BiDi markers consistently (as it does for regular `/* ... */` comments).
Actual: no directional-override error for the NatSpec case.

This is also accepted because `validateBiDiMarkup()` does not track isolate markers.