feat: add SCROLL statement for pagination support in QQL

[srimon12]

Summary

This PR adds first-class pagination support to QQL via a new SCROLL statement.

Supported syntax:

SCROLL FROM docs LIMIT 50
SCROLL FROM docs WHERE year >= 2024 LIMIT 50
SCROLL FROM docs AFTER 'cursor-id' LIMIT 50
SCROLL FROM docs WHERE year >= 2024 AFTER 42 LIMIT 50

This closes a usability gap for browsing large collections and paginated exports using SDK-native scroll behavior.

Why

• Qdrant SDK already exposes scroll(...)
• QQL already uses scroll(...) internally in dumper flows
• Users need a direct query-level pagination primitive for browsing and operational workflows

Changes

Language / Parsing

• Added SCROLL as a top-level statement
• Added AFTER clause support for cursor-based pagination
• Added ScrollStmt AST node with:
  • collection
  • limit
  • optional query_filter
  • optional after cursor (string or integer)

Execution

• Added _execute_scroll(...) in executor using QdrantClient.scroll(...)
• Supports:
  • optional filter (scroll_filter)
  • optional cursor (offset)
  • payload-only fetch (with_payload=True, with_vectors=False)
• Returns structured result:
  • points: list of {id, payload}
  • next_offset: cursor for the next page (or None)

CLI / Script

• Added SCROLL help text in CLI command reference
• Added pretty-print output for SCROLL points + next_offset
• Added SCROLL to script statement splitting so .qql scripts can execute SCROLL statements cleanly

Documentation

Updated docs and examples to include SCROLL:

• README quick syntax + feature summary
• search docs (SEARCH, SCROLL, RECOMMEND...)
• programmatic usage result shape
• getting started examples
• reference error table
• docs homepage card text

Tests

Added/updated tests for:

• lexer: SCROLL/AFTER tokenization
• parser:
  • basic SCROLL
  • SCROLL + WHERE
  • SCROLL + AFTER
  • SCROLL + WHERE + AFTER
• executor:
  • successful scroll result shape
  • forwarding filter and cursor
  • missing collection error
• script splitter:
  • SCROLL recognized as top-level statement boundary

Validation

• Focused test suites passed
• Full suite passed:

pytest -q
# 409 passed