Clarify BloomFilter semantics after invert and correct builder documentation

[tisonkun]

Three related BloomFilter contract/documentation issues should be resolved together:

1. invert flips the backing bit array and therefore the resulting object no longer satisfies the normal Bloom-filter no-false-negative guarantee advertised at the type and module level.
2. is_empty is documented in terms of inserted items, but after invert it only reports whether the backing bit array has zero set bits.
3. BloomFilterBuilder::build documents a panic when neither accuracy nor size was configured, but that state is not constructible through the public API: the builder has no public default constructor and both public entry points initialize a valid configuration.

Design questions:

• Should invert remain on the normal BloomFilter API, move to a raw/diagnostic API, or make the altered guarantee observable in the type/state?
• Should is_empty retain a logical-stream meaning, be renamed to describe bit state, or document mode-dependent semantics?
• Which module/type/method guarantees need a post-invert qualification?
• Remove the unreachable build panic documentation and, if possible, encode the configured builder state more directly.

This issue intentionally excludes adding try_union/try_intersect; that is a separate fallibility/API-consistency decision.