Detect duplicate keys and overwrites in inline tables - #100

Add key-path tracking and 4-step validation to `toml:read-inline-table`:

Three tracking lists are maintained across the while loop:
- defined-paths: leaf paths that have been assigned a value
- implicit-paths: intermediate paths created by dotted keys
- inline-table-paths: paths whose value is an inline table (alist)

Validation steps (executed in order before storing each key-value):

1. Exact duplicate: full-path already in defined-paths
   e.g. {b=1, b=2}
2. Implicit table overwrite: full-path is a proper prefix of
   an existing defined-path (would destroy children)
   e.g. {b.c=1, b=2}
3. Inline table immutability: a proper prefix of full-path is
   in inline-table-paths (TOML spec forbids extending inline tables)
   e.g. {inner={dog="best"}, inner.cat="worst"}
4. Nesting into scalar: a proper prefix of full-path is in
   defined-paths but not in implicit-paths or inline-table-paths
   e.g. {b=1, b.c=2}

Steps 1/2 check defined-paths (any value type), while step 3
specifically checks inline-table-paths (alist values only).
Step 4 catches the reverse direction of step 2 (child under scalar).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>