… on faster .prx windows #69769 (#69796)

### What problem does this PR solve?

Issue Number: None

Related PR: #69769, #69649 (backport #69795), #69770 (backport #69788)

Problem Summary: Backport of #69769 to branch-4.2, stacked on #69795
(the backport of #69649). #69649 stores a .prx window as PFOR instead of
zstd when zstd saves less than `snii_prx_zstd_min_saving_percent` (30)
of the PFOR frame, because every read of a zstd window pays a
decompression. What that costs and buys depends on how much zstd saves
on the data. Replaying the 1.6M windows of 10M AgentLogs rows, 30 spends
8% more bytes on the windows zstd compresses and cuts their decode time
by 42%; on the windows of TextBench service logs it spends 5% and cuts
6%; on data whose windows zstd shrinks by just under 30% it stores every
window as PFOR. No single percentage fits all data.

Each index of a segment now spends a size budget instead:
`PrxZstdBudget`, owned by its `LogicalIndexWriter` and reached through
`PrxWindowLimits`, so the resident and the spilled encoder, and the
ordinary and the compaction build, all use it. A window whose smallest
frame is zstd is stored in its faster frame (PFOR, or raw when smaller)
when
- its saving is below the share whose trades, over the windows the index
has seen, fit 8% of their smallest frames, which puts the budget on the
windows zstd shrinks least, and
- the bytes the index has actually traded, this window included, still
fit 8% of the smallest frames seen. This makes 8% a bound for any order
of windows: fed the TextBench windows from the largest saving to the
smallest, the share alone spends 36%.

Until it has seen 64 windows the share is 30, the previous default. The
windows counted are those that get a zstd frame (a plain payload of at
least 512 bytes), the only ones whose codec a share decides; singleton
and short windows are written as before and counted neither in the
allowance nor in the warmup. The choice uses the frames the writer
builds anyway: nothing is compressed twice or scanned ahead, and the
same input gives the same bytes. `snii_prx_zstd_min_saving_percent` now
defaults to -1 (the budget); 0-100 keeps a fixed share, 0 the smallest
frame.

The budget is 8% rather than less: with it the AgentLogs phrase queries
below read nearly the same frames as with 30, while in the replay a 5%
budget cuts the decode time by 31% instead of 39%.

Same binary, one stream load per table, size against the smallest frames
(0):

| data | index, 30 | index, budget | table, 30 | table, budget |
|---|---|---|---|---|
| AgentLogs 99M rows | +4.13% | +3.65% | +1.92% | +1.69% |
| AgentLogs 10M rows | +4.59% | +3.37% | +2.13% | +1.57% |
| TextBench 12M rows | +0.89% | +0.79% | +0.51% | +0.45% |

- The phrase queries read the same frames with 30 and with the budget:
at 99M rows all PFOR for DS1, DS6, DS7, A6 and A9 (for example 107,307
frames for DS7), and about 11,170 zstd frames of about 259,000 for A10
with both; on TextBench most windows stay zstd either way (582 against
583 zstd frames of 599 for 'connection reset').
- Hot queries in one process, budget data against 30 data with the same
read path (paired cycles: 6 for the 25 AgentLogs queries at 10M rows, 10
for the 15 TextBench queries): 0.96-1.06 and 0.97-1.05; every query
returns the same result.
- Against the data written before #69649 (smallest frames), the 11
AgentLogs phrase queries at 99M rows take 0.83x in geometric mean with
the budget data (DS7 0.63x, A6 0.67x, A9 0.70x, DS4 0.74x, A10 0.77x),
as with 30 (0.83x). This run also had the read-path change of #69770,
which does not touch the PFOR windows these queries read.
- Data bytes are identical, and the index written with 0 and with 30 has
exactly the size the previous binary writes.
- Write cost: the choice adds about 48 ns per window, 79 ms over the
1.6M AgentLogs windows.

### Branch adaptations

None. `git cherry-pick 0571eb6` applied cleanly on top of the
backport of #69649, which it needs: it changes the setting #69649 adds,
and the automatic pick to branch-4.2 failed because branch-4.2 does not
have #69649. This PR is stacked on that backport, #69795, so it shows
both commits until #69795 merges; merge #69795 first.

### Validation

- Built and run with both backports (this one and the stacked #69769
backport) applied to branch-4.2 at 711db43, the commit before #69625:
its lance-c update is not in the local third-party build, and the 16
commits after it touch no SNII file. `PrxPod*`, `Snii*`,
`DorisSniiFileReader*`, `*InvertedIndex*`, `*IndexFileWriter*` and
`*IndexFileReader*`: 1710 tests passed, including the new budget, codec
choice, writer and configuration tests.
- On master both changes passed CI.
- Not built locally on the branch-4.2 tip; this PR's BE UT run covers
that.

### Release note

SNII now chooses per index how many position windows to store in the
faster but larger PFOR form, spending at most 8% of the size of the
windows zstd compresses; `snii_prx_zstd_min_saving_percent` defaults to
-1 for this, and 0-100 still sets a fixed percentage.

### Check List (For Author)

- Test: Unit Test
- SniiPrxZstdBudget: the prior share, the windows zstd shrinks least are
traded first, the bound under a descending order, windows with nothing
to trade.
- SniiPrxPodTest: the faster frame (PFOR or raw) and when the budget is
asked.
- SniiStreamedWriterSessionTest: windows that save 1-29% through the
ordinary and the streamed (compaction) build; 30 trades all of them, the
budget at most 8% of the smallest frames.
    - SniiWriterPrxZstdSaving: the default and the config mapping.
- Manual test: index size of 10M and 99M AgentLogs and 12M TextBench
loads with 0, 30 and the budget; frames read per codec from query
profiles; interleaved hot A/B in one process.
- Behavior changed: Yes (the default .prx codec choice of newly written
segments; existing segments are read as before)
- Does this need documentation: No

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: martin.foo838 <[email protected]>
Co-authored-by: Claude Opus 5.5 <[email protected]>