feat: implement LEAD() and LAG() window functions (#1216)

[imsanupm]

Description

Partial fix for #1216

Implements LEAD() and LAG() window offset functions with full
PARTITION BY and ORDER BY support.

Syntax Supported

LEAD(col, offset, default) OVER (PARTITION BY col ORDER BY col)
LAG(col, offset, default) OVER (PARTITION BY col ORDER BY col)

Changes

• src/424select.js — detect LEAD/LAG functions and store config
  in query.windowoffsets
• src/40select.js — post-process LEAD/LAG after query execution,
  grouping rows by partition and applying offset logic
• src/55functions.js — register LEAD/LAG as stub functions
  (actual logic handled in post-processing)
• test/test-1216-lead-lag.js — regression tests covering LAG,
  LEAD, default values, and custom offsets

Tests

• 4 new regression tests all passing
• All 2449 existing tests still passing

[mathiasrw]

It looks like you have pushed the wrong commit.