Add match-count builtin: count matches without materializing results

[Tsion-Teklay]

Summary

This PR adds a match-count builtin that efficiently counts the number of atoms matching a pattern without materializing the entire result set. It addresses a critical performance issue where counting matches using (length (collapse (match ...))) would cause stack overflow on large knowledge bases.

Problem

Counting how many atoms in a space match a pattern is currently done via (length (collapse (match ))), which builds a Prolog list of every match. On large in-RAM AtomSpaces this exhausts the stack long before the count is needed — the caller only wants a number, not the results themselves.

Solution

Implemented match-count using aggregate_all/3 which is failure-driven and keeps a single counter — no list is built. The new function:

• Uses constant memory regardless of result size
• Reuses the existing match/4 implementation
• Returns the count directly without materializing matches

Implementation

match-count(Space, Pattern, Count) :-  
    aggregate_all(count, match(Space, Pattern, _, _), Count).

The function is added to src/spaces.pl and registered in src/metta.pl to enable MeTTa calls.

Testing

Added comprehensive tests in examples/match_count.metta covering:

Full pattern count: (match-count &self (parent $p $c)) => 5
Bound pattern count: (match-count &self (parent alice $c)) => 2
Zero matches: (match-count &self (parent zelda $c)) => 0

Test with:
sh run.sh ./examples/match_count.metta

Expect: three lines each ending in ✅

Files changed

src/spaces.pl — defines match-count/3 as a wrapper over aggregate_all(count, match(...), Count)
src/metta.pl — registers match-count in the function list
examples/match_count.metta — three test cases