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):

• 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 [improvement](be) Let SNII skip zstd for .prx windows where it saves little over PFOR #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 [improvement](be) Decode SNII zstd and raw .prx windows faster #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 0571eb6a58 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 [improvement](be) Let each SNII index spend a size budget on faster .prx windows #69769 backport) applied to branch-4.2 at 711db43, the commit before [enhancement](lance) Add cache diagnostics and BE runtime index selection #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

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