fix: encode descending sort columns correctly in sorted_series key (#6338)

* fix: encode descending sort columns correctly in sorted_series key

The sorted_series key uses storekey for order-preserving binary encoding,
but storekey always encodes ascending. For sort schemas with descending
pre-timestamp columns (e.g. "-service"), the encoded bytes must be
bitwise-NOTed so that ascending memcmp on the composite key gives the
correct descending order. This is the standard ordered-code technique
(see Google's OrderedCode).

Without this fix, a descending pre-timestamp column would cause
sorted_series bytes to not be monotonically ascending in the file,
breaking DataFusion streaming aggregation assumptions and merge
correctness.

Changes:
- KeyColumn now carries `descending: bool` from the parsed sort schema
- encode_row_key inverts storekey bytes for descending columns
  (ordinal + value together)
- Added invert_bytes helper
- timeseries_id is always ascending (it's a hash tiebreaker)

Test: ascending schema gives alpha < zebra in key bytes; descending
schema gives zebra < alpha (inverted). Keys from different schemas
differ for the same data.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: only invert value bytes for descending columns, not ordinal

When a descending sort column has a null value, the null row skips
the column entirely in the key. If the ordinal bytes were inverted
(!ordinal = 0xFE), null rows (whose next byte is the small
timeseries_id ordinal, e.g. 0x02) would sort before non-null rows,
violating the writer's nulls_first=false behavior.

Fix: write the ordinal normally (ascending) and only invert the
storekey-encoded value bytes. This ensures:
- Non-null descending: ordinal(small) + !value
- Null (skipped): next ordinal(slightly larger)
- So non-null keys < null keys, matching physical nulls-last order

Added test: descending service with null — verifies
key(beta) < key(alpha) < key(null).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: reject sort schemas with tag columns after timeseries_id

The sorted_series key encodes columns up to (and including)
timeseries_id. Any tag column after timeseries_id would be in the
physical sort order but not in the key, breaking merge correctness —
two rows with the same timeseries_id but different extra_tag values
would have identical sorted_series keys.

Added validation in validate_schema: after timeseries_id, only
timestamp and tiebreaker may follow. Any other column is rejected
at parse time.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: g-talbot