Add --list-operations flag; rewrite Multi-Source Rules README section

[matthewhorridge]

Summary

• harmonize --list-operations prints each available primitive operation with the first paragraph of its class docstring, so the listing can never drift from the implemented primitives
• The factory's 50-line match statement is replaced by an explicit OPERATION_CLASSES registry that both deserialization and the listing dispatch through
• --rules is no longer required by argparse; it is enforced in main() for the validate and harmonize modes
• Rewrites the Multi-Source Rules README section in plainer language: leads with the two concrete data situations (value in different columns depending on units; one measurement split across columns), walks through each YAML example row by row with inline comments, and splits case / coalesce / multi-operand branches into their own subsections

Sample output:

bin
  Assign values into histogram bins. Performs a range query using an interval
  tree. Bins must not overlap.

case
  Choose one of several branches by switching on a selector source, then
  compute that branch's value from one or more source operands.
...

Test plan

• 6 new tests: registry covers the full vocabulary, every class has help text, list mode output, and --rules enforcement for both remaining modes
• pytest tests — 221 passed
• All complete YAML rule examples in the README (including the new commented ones) load through RuleSet.load

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