feat(vfs): add observability PRAGMAs and relative time parsing

[corylanou]

Summary

This PR adds VFS observability features for monitoring replica health and enhances time travel functionality. It provides both SQL functions and PRAGMAs for maximum flexibility:

• litestream_txid (feat(vfs): implement litestream_txid() SQLite function #851): Return current transaction ID
• litestream_lag (feat(vfs): implement litestream_lag() SQLite function #852): Return seconds since last successful poll
• litestream_time enhancement (feat(vfs): extend litestream_current_time() to show latest LTX timestamp #853): Return actual LTX timestamp instead of "latest" when viewing current data
• Relative time parsing (feat(vfs): add relative time parsing to litestream_set_time() #854): Accept human-readable relative time expressions like "5 minutes ago" or "yesterday" using go-dateparser

SQL Functions (via extension auto-extension)

-- Get current transaction ID
SELECT litestream_txid();  -- Returns: 1

-- Get seconds since last successful poll
SELECT litestream_lag();  -- Returns: 2

-- Get current view time
SELECT litestream_time();  -- Returns: 2024-01-15T10:30:00.123456789Z

-- Set time travel with relative expressions
SELECT litestream_set_time('5 minutes ago');
SELECT litestream_set_time('yesterday');
SELECT litestream_set_time('LATEST');  -- Reset to current

PRAGMAs

-- Get current transaction ID
PRAGMA litestream_txid;  -- Returns: 1

-- Get seconds since last successful poll
PRAGMA litestream_lag;  -- Returns: 2

-- Get/set current view time (now returns actual timestamp instead of "latest")
PRAGMA litestream_time;  -- Returns: 2024-01-15T10:30:00.123456789Z

-- Set time travel with relative expressions
PRAGMA litestream_time = '5 minutes ago';
PRAGMA litestream_time = 'yesterday';
PRAGMA litestream_time = '2 hours ago';
PRAGMA litestream_time = LATEST;  -- Reset to current

Changes

• vfs.go: Add new PRAGMA handlers, fields for tracking state, and time parsing helper
• vfs_connections.go: Connection-to-VFSFile mapping for SQL function support
• src/litestream-vfs.c: SQL function implementations and auto-extension registration
• cmd/litestream-vfs/main.go: CGO exports for SQL functions
• go.mod: Add github.com/markusmobius/go-dateparser dependency
• cmd/litestream-vfs/time_travel_test.go: Add/update tests

Test Plan

• Existing VFS tests pass
• New PRAGMA tests for litestream_txid, litestream_lag, and relative time parsing
• Pre-commit hooks pass

Documentation

Created issue benbjohnson/litestream.io#146 to document these features.

Closes #851, #852, #853, #854

[benbjohnson]

@corylanou Can you have the litestream_txid PRAGMA & function return the TXID in hex format? There's an ltx.FormatTXID() that'll format it correctly. That's pretty much how every TXID is formatted. We could add a separate litestream_txid_int function in the future if someone wants the raw number but the formatted version is generally more useful.

[corylanou]

@corylanou Can you have the litestream_txid PRAGMA & function return the TXID in hex format? There's an ltx.FormatTXID() that'll format it correctly. That's pretty much how every TXID is formatted. We could add a separate litestream_txid_int function in the future if someone wants the raw number but the formatted version is generally more useful.

Done!