Raise a helpful error for {{ index }} outside an items expansion (fixes #186)

[fcsonline]

Fixes #186.

Problem

{{ index }} is only seeded into the context inside with_items, with_items_range, with_items_from_csv and with_items_from_file requests, where it is the item's position in the list. Referencing it anywhere else (e.g. a plain request, or an assert on index) failed with a generic, unhelpful message:

• panic in the default strict mode: Unknown 'index' variable!
• a bare warning under --relaxed-interpolations

Neither gave any clue that index is scope-limited, and the user almost always actually wanted the iteration counter — which already exists as iteration.

Fix

Rather than overloading index with a second meaning, detect this specific case and raise a dedicated message that explains the variable's scope and points to iteration:

Unknown 'index' variable! It is only available inside 'with_items',
'with_items_range', 'with_items_from_csv' or 'with_items_from_file' requests.
Use 'iteration' for the current iteration number.

The message is used both for the strict-mode panic and the relaxed-mode warning. with_items* requests are unaffected — index still resolves there as before.

Docs

Documented the built-in interpolation variables (base, iteration, item, index) and index's scope in SYNTAX.md.

Verification

• Issue {{ index }} not working #186 repro now fails with the helpful message instead of Unknown 'index' variable!.
• A with_items request still resolves {{ index }} (e.g. /a/0, /b/1).
• Relaxed mode prints the helpful warning and continues.
• New unit test index_outside_expansion_suggests_iteration. cargo fmt, cargo clippy -- -D warnings, cargo test (62) all pass.