fix: support multiple file handles to the same mocked file (#27)

[Koan-Bot]

Summary

Fixes #27 — allows opening multiple independent file handles to the same mocked file.

Problem

The %files_being_mocked hash tracked a single fh per mocked path. Opening a second handle to the same file overwrote the first, and closing any handle cleared tracking entirely.

Solution

• Replace single {'fh'} with {'fhs'} arrayref of weak references
• Each open/sysopen pushes to the array instead of overwriting
• CLOSE removes only the specific handle (matched by tied object identity)
• _fh_to_file() scans all entries, cleaning dead weakrefs during traversal
• Directory handles (opendir/closedir) are unchanged — they use a separate pattern

Test plan

• New t/multi_handle.t with 4 test groups:
  • Two read handles reading independently
  • Read + append handles sharing contents
  • Handle tracking (fhs array count after open/close)
  • Sysopen dual handles
• Updated t/readline.t assertion for new fhs structure

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