mdbplyr 0.4.0

This release expands the lazy MongoDB translation layer introduced in v0.3.0, moving mdbplyr from a small core verb subset to a broader analytical backend with joins, window functions, richer summaries, and schema-aware selection helpers.

New verbs and query translations

• Added inner_join(), left_join(), semi_join(), and anti_join() methods for tbl_mongo.
• Joins compile to MongoDB $lookup pipelines. Mutating joins flatten matches by default with $lookup, $unwind, and $replaceRoot; unnest = FALSE keeps matches as a nested array column.
• Filtering joins keep the left-hand columns and filter rows by whether a match exists.
• Join support is intentionally conservative: the right-hand side must be a plain tbl_mongo collection reference with a known schema in the same database.

Expanded select() and expression support

• select() now supports name-based tidyselect helpers when the schema is known: starts_with(), ends_with(), contains(), matches(), everything(), all_of(), any_of(), ranges, and negation.
• select() still rejects where() because column types are unknown without reading data.
• coalesce() is now translated in scalar expressions and compiles to nested MongoDB $ifNull expressions.
• row_number() is available as an alias for the existing 1:n() row-numbering idiom in mutate() and transmute().

Grouping and summaries

• group_by() now supports named computed grouping keys, such as group_by(bucket = floor(amount / 10)).
• summarise() gained sd(), var(), first(), last(), and n_distinct().
• median() and quantile() are supported on MongoDB 7.0 or newer through MongoDB’s percentile accumulator.
• Summary expressions now use the same field-vs-local name resolution rules as other translated expressions.

across()

• Added across() support in mutate() and summarise().
• Supported selections are name-based and schema-aware, matching the new select() helper support.
• Supported functions are bare function names and ~ lambdas, with optional .names templates.
• across() deliberately does not support where(), anonymous functions, or functions stored in variables.

Window functions

• Added window function translation in mutate() and transmute() via MongoDB $setWindowFields.
• Supported ranking functions are rank(), min_rank(), and dense_rank().
• Supported cumulative functions are cumsum(), cummean(), cummax(), and cummin().
• Supported offset functions are lag() and lead().
• Window functions require MongoDB 5.0 or newer. Ranking functions sort by their column argument; cumulative and offset windows take their order from a preceding arrange().

Server version awareness

• mongo_src() and tbl_mongo() now accept an optional server_version argument.
• When possible, mongo_src() probes the connected MongoDB server with buildInfo.
• Added mongo_server_version() for mongo_src and tbl_mongo objects.
• Version-gated features now fail clearly on servers that are known to be too old.

Behavior fixes and internals

• mutate() assignments now compile in user order, so later expressions can refer to fields created earlier in the same call.
• Fixed a select() regression involving nested root paths.
• Pipeline compilation now preserves operation order more explicitly through the internal query representation, which is needed for ordered mutation, row-numbering, window functions, and joins.
• Added extensive mock-backed tests for the new phases and for flat, nested, and missing-field semantics.

Documentation and packaging

• The README and vignette now document the v0.4.0 feature set, limitations, and translation model.
• Installation instructions were updated for CRAN availability while preserving GitHub installation instructions for development builds.
• Generated vignette artifacts were removed from the source repository.

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