security: MountableFs missing validate_path(), misc VFS gaps (TM-DOS-046, TM-DOS-049, TM-DOS-050)

[chaliy]

Summary

Several lower-severity VFS and parser issues found during the deep audit:

TM-DOS-046: MountableFs at fs/mountable.rs:348-491 has zero validate_path() calls across all FileSystem trait methods. Path depth/character validation is completely bypassed for mounted filesystems.

TM-DOS-049: collect_dirs_recursive at interpreter/mod.rs:8352 has no explicit depth limit when recursing through VFS directory trees. Mitigated by max_path_depth in FsLimits but no direct protection.

TM-DOS-050: parse_word_string at parser/mod.rs:109 creates a parser with default limits, ignoring caller-configured tighter limits. Parameter expansion in word contexts gets unlimited parser operations.

Impact — LOW to MEDIUM

• TM-DOS-046: Path validation gaps in mounted filesystems
• TM-DOS-049: Theoretical deep recursion DoS (mitigated by existing limits)
• TM-DOS-050: Parser limit bypass in parameter expansion

Recommended fix

• Add validate_path() to all MountableFs FileSystem methods
• Add explicit depth parameter to collect_dirs_recursive
• Propagate limits through parse_word_string

Cross-references

• Threat model: TM-DOS-046, TM-DOS-049, TM-DOS-050
• PR: test(security): deep security audit with regression tests #487