fix: clamp lag parameter to buffer size with warning (fixes #379)

[GaneshPatil7517]

Summary

Fixes #379 — lag parameter in plotymlag.py silently wraps around when lag >= buffer size, producing incorrect data without any warning.

Root Cause

The circular buffer in plotymlag.py uses a fixed size = 10. The lag value is user-configurable via concore.tryparam("lag", 0). When a user requests lag >= size (e.g., lag = 15), the expression (cur - lag) % size silently wraps due to Python's modulo arithmetic, causing lag = 15 to behave identically to lag = 5. This produces wrong output data with no error or warning.

Fix

• Added input validation immediately after lag is read from tryparam.
• When lag >= size, the value is clamped to size - 1 (the maximum safe lag).
• A logging.warning() is emitted to alert the user that their requested lag was out of range.
• The existing circular buffer logic ((cur - lag) % size) remains unchanged.

Changes

• tools/plotymlag.py: Added import logging and a validation block (6 net new lines).

Testing

Verified locally with multiple lag values:

Input lag	Effective lag	Warning emitted
0	0	No
5	5	No
9	9	No
10	9	Yes
15	9	Yes
100	9	Yes

• Syntax check passes (py_compile).
• Backward compatible: existing users with lag < 10 see no change in behavior.

Verification Steps for Reviewers

1. Apply patch and run plotymlag.py with lag=0 → works as before.
2. Run with lag=15 → observe warning message and clamped behavior (lag=9), no crash.
3. Confirm (cur - lag) % size logic is untouched.
4. Confirm no new dependencies are introduced (only stdlib logging).