feat(system): environment() table function (S9)

[TimelordUK]

Summary

Adds environment() beside processes() and sockets(): one row per environment variable the sql-cli process inherited.

SELECT * FROM environment() WHERE UPPER(name) LIKE 'CARGO%';

WITH e AS (SELECT value FROM environment() WHERE UPPER(name) = 'PATH')
SELECT UNNEST(value, ';') AS directory FROM e;   -- ':' on Unix

• Columns name, value; the same on every platform, sorted case-insensitively by name.
• Windows' hidden per-drive =C: variables are skipped.
• An empty value stays '' rather than NULL, because on Unix set-but-empty is different from unset.
• Non-UTF-8 values are converted lossily, never dropped.
• Named environment() rather than env() to keep ENV('X') free for a scalar lookup.
• Portability: Windows stores names case-insensitively, and the spelling follows the launching shell (Path from PowerShell, PATH from Git Bash). Use UPPER(name) in portable queries.

Docs: S9 in docs/SYSTEM_TABLES.md; examples in examples/system_environment.sql.

Test plan

• Unit test environment_lists_what_this_process_inherited passes on Windows 11 and on Linux (WSL2); all system tests pass on Linux
• On Linux, FOO= returns '' with length 0, and PATH splits on :
• cargo clippy: no new warnings
• ./run_all_tests.sh passes. One smoke test, expander_rewriters, fails on a GROUP BY error; it also fails without this change.

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