refactor(api)!: one name per meaning across the public API

[rducom]

Intent

Give the public API one vocabulary. A review of every public member found names that said something other than what the member does, words used for two things (Statistics for the data and for what a query did, Append for writing rows and for opening a writer), abbreviations, and a few members that broke rules the rest of the API keeps. The branch renames those members, writes the rules down in the public API design, and lists every rename for callers.

What it brings

• One name per meaning. A scan's aggregates are AggregateAsync, next to CountAsync and SumAsync, and an aggregator of one's own stays an overload of the same name. What a query did is Metrics (ScanMetrics, GroupMetrics), and statistics only describe the data. A writer over an existing file comes from OpenWriterAsync, the pair of CreateWriter. A dataset lists, removes and replaces objects (ListObjectsAsync, RemoveObjectsAsync, ReplaceObjectsAsync), and deletes and updates rows.
• Names that say what they return. CountAtKeyAsync counts the entries of the current key, and IndexOf(utf8) returns an index, -1 when no field matches.
• One meaning per prefix. As is the same data in another form, still borrowed, so the four forms of a column read alike (AsCanonical, AsDictionary, AsRunEnd, AsConstant) and AsStorage joins them. To is an owned copy, and Get a value computed or read (GetValidLengthAsync).
• No abbreviation (PreviousAsync, PreviousKeyAsync, SeekMode), quantities of bytes that say so (MaxDecompressedBytes, InitialReadBytes, CapacityBytes, HeldBytes), and switches that are verbs (UseStatistics, WriteStatistics, UseLeases).
• The rules in a new "Names" section of the public API design, with a table of every rename. The query design's migration table and the queries guide point to it.

Risks and tests

• The change breaks every caller of a renamed member. Nothing else moves: no byte on disk, no format constant, no default, no behaviour.
• dotnet build Vorticity.slnx -c Release passes with no warning.
• Vorticity.Tests passes in full, 7 311 tests, with 4 skipped by their own environment conditions. Vorticity.Conformance passes, 923 of 923. The MustNotCompile cases raise VX1010 and VX1011 under the new name.
• The Zstd tests and the bench were not run, since neither the managed zstd code nor any executed path changed.

Every rename, before and now

before	now
scan.AggAsync(a => e), scan.AggAsync<TResult>(a => (…))	scan.AggregateAsync(a => e), scan.AggregateAsync<TResult>(a => (…))
Statistics on a scan, a projection and an aggregation, ScanStatistics, GroupStatistics	Metrics, ScanMetrics, GroupMetrics
session.AppendAsync(path)	session.OpenWriterAsync(path)
dataset.ObjectsAsync(), RemoveAsync(objects), ReplaceAsync(removed, added)	ListObjectsAsync(), RemoveObjectsAsync(objects), ReplaceObjectsAsync(removed, added)
cursor.KeyCountAsync(), PrevAsync(), PrevKeyAsync(), SeekOp	CountAtKeyAsync(), PreviousAsync(), PreviousKeyAsync(), SeekMode
schema.TryGetField(utf8, out int index)	schema.IndexOf(utf8), -1 when no field matches
column.Canonical(), column.Storage()	column.AsCanonical(), column.AsStorage()
VortexFileRepair.ValidLengthAsync(path)	VortexFileRepair.GetValidLengthAsync(path)
MaxDecompressedSize, MaxBatchDecompressedSize and InitialReadSize on VortexOpenOptions, VortexLimits.DefaultMaxDecompressedSize	MaxDecompressedBytes, MaxBatchDecompressedBytes, InitialReadBytes, DefaultMaxDecompressedBytes
SegmentCache.Capacity, SegmentCache.Size	CapacityBytes, HeldBytes
ScanOptions.Pruning, VortexWriteOptions.Statistics, CompactionSchedule.Leases	UseStatistics, WriteStatistics, UseLeases
CompactionJob.Objects, GroupKeyPlan.Values, VortexDataset.ClusteringKeyPaths	ObjectKeys, ValueRange, ClusteringKey

Kept as it is, on purpose

• Counts keep their names, RowCount, ZoneCount and EntryCount next to Rows and Entries. Turning every count into one form would rename compound names such as BlocksPruned or ObjectsIn badly.
• TimeUnit keeps its plurals (Milliseconds, Days), since a unit that counts a quantity is plural, as in TimeSpan.FromMilliseconds. CalendarUnit stays singular, since it names the period Truncate rounds down to.
• VortexFileFormat.InitialReadSize and VortexLimits.MaxPostscriptSize are constants of the Vortex format and keep its names.
• Sym, Expr and DType are the three abbreviations left, terms of the domain a reader meets on every page.
• The Vortex prefix stays as it is. No rule would make it regular without renaming some thirty types.

Inside the library

• An internal type was already named ScanMetrics: the counters a scan adds to, whose snapshot the public ScanStatistics was. It and DatasetScanMetrics became ScanCounters and DatasetScanCounters, and the internal members that hold them are named Counters, beside Metrics for the snapshot.
• The internal read options follow the public open options into …Bytes, and the one error message that named the internal type now names VortexOpenOptions.MaxDecompressedBytes.
• The parameter of KeyCursor<TKey>.SeekAsync is mode, after its type. The internal cursors keep op.
• Three printed sample labels follow the new names, and the guide shows the two it quotes.

Commits

1. The first part: the names that said something else, or used one word for two things.
2. The second part: prefixes, abbreviations, bytes, switches and the remaining one-word-two-meanings.
3. The rules and the table of renames in the public API design.
4. A sample label that still printed Canonical().