AddFiles: PathPattern and RouteFiles - #40749

Adds the two self-contained pieces that dynamic destinations for IcebergIO.AddFiles are
built on: a path-pattern matcher, and the DoFn that uses it to name a destination table
from each file's path.

AddFiles registers every input file into one table named by table. Migrating a lake
usually means a tree of per-table folders, which today needs one AddFiles per table.
The goal is for one transform to route each file to the table its path names:

path_pattern:  gs://lake/export/{table}/**
table:         default.{table}

gs://lake/export/orders/part-0.parquet     ->  default.orders
gs://lake/export/customers/part-0.parquet  ->  default.customers

This PR adds only the routing primitives so they can be reviewed on their own. The
pre-pass, registration and schema-transform surface follow separately.

PathPattern

A whole-path glob with {name} placeholders, each capturing exactly one path segment.
Globbing is Beam's own (FileSystemUtils.wildcardToRegexp), so *, ** and ? behave as
they do in any other Beam file pattern; placeholders are translated to capturing groups
between the globbed fixed parts.

The whole path must match, so the pattern decides both which table a file belongs to and
whether it belongs at all. compile rejects patterns that cannot answer that
unambiguously:

render(template, captures) fills the table template, and checkRenderable(template)
refuses a template naming a placeholder the pattern never captures, so table: default.{db} against .../{tbl}/** fails at construction rather than producing one error
row per file.

RouteFiles

DoFn<String, KV<String, String>> with two outputs:

• ROUTED: KV<table, path> for a file the pattern matched
• ERRORS: a {file, error, table} row for a path it did not, with table empty because
  there is no table to attribute it to

An unmatched path becomes an error row rather than being silently dropped: a mistyped
pattern must be visible. PathPattern.compile and checkRenderable both run in the
constructor, so a bad pattern or template fails at pipeline construction instead of once
per file on a worker.

• RouteFiles.ERROR_SCHEMA is declared locally here. When the transform is wired up it
  becomes the same schema as AddFiles' error output, so the two can be flattened; that
  change belongs with the wiring.
• Patterns are interpreted by Beam's Java glob translator. Note it normalises **/* to
  **, which Python's FileSystem.translate_pattern does not. Relevant only if you
  compare a path_pattern against a glob written for Python's MatchFiles upstream.