Add column-scoped search predicates (Fixes #776)

[DannyStoll1]

Summary

Adds column-scoped search predicates, giving a general way to filter by a single field. This resolves #776 ("filter processes of a user") without a special-purpose flag:

procs user==root         # only root's processes
procs command~=dockerd   # command contains "dockerd"

Behavior

A search keyword now parses into either a plain keyword (matched against every searchable column, as before) or a <column><op><value> predicate:

• == : exact, full-string match
• ~= : substring match

Details:

• The column name resolves case-insensitively against the existing column kinds (a substring is enough, e.g. user, command, pid).

• A predicate matches its target column regardless of that column's numeric_search / nonnumeric_search flags, so pid==1 works even when Pid is not otherwise searchable.

• Only the value after the operator may contain further = characters; the leftmost operator splits the token.

• Predicates are ordinary search terms, so they compose with plain keywords and with --and / --or / --nand / --nor:

  procs --and user==root command~=dockerd

• Unknown column names, and columns absent from the active layout, are reported as errors before any output.

• Predicate parsing is disabled in --regex mode, where the pattern is taken verbatim.

Implementation

View::filter's matching is extracted into a Search type that compiles the keywords once (regex, or a set of per-column terms) and exposes matches(pid). Plain-keyword matching folds per-term results with the active logic; this is equivalent to the previous numeric/nonnumeric group combine, verified for all four logic modes. View::filter shrinks from ~200 lines to ~70.

Tests

• Unit tests for predicate parsing (operators, value containing =, unknown/empty column).
• Integration tests in run_default mirroring the existing search tests, plus error cases.
• Existing search tests pass unchanged.

Docs updated: README (new subsection + feature bullet), man page, and CHANGELOG.