feat: put bulk-load batch progress behind a debug flag

[jvendetti]

Problem

append_triples_batch printed a progress line for every batch, unconditionally. Running a single unit test in a dependent gem (ontologies_linked_data) produces dozens of them:

TestAncestorsPrecompute#test_memoization = 0.00 s = .
Appending triples in batch of 146449 triples from line 0
Appending triples in batch of 5 triples from line 0
Appending triples in batch of 1698 triples from line 0
... (dozens more)

The information is useful when debugging a slow or failing ontology load, and noise everywhere else.

Change

Adds a data_load_debug flag, following the shape of the existing queries_debug switch directly above it:

• Goo.data_load_debug / Goo.data_load_debug? in lib/goo.rb
• GOO_DATA_LOAD_DEBUG env var, plumbed through config.rb and documented in config.rb.sample
• the puts at lib/goo/sparql/client.rb:189 is now guarded by it

Off by default in lib/goo.rb itself, so callers that never reach Goo.config — unit tests in dependent gems, notably — get the quiet default with no change on their side. Opt back in with GOO_DATA_LOAD_DEBUG=true or a direct Goo.data_load_debug(true).

Notes for review

• The setter is positional (data_load_debug(flag)) rather than data_load_debug=, because it is called inside a Goo.configure block where conf.x = v would parse as a local variable assignment rather than a method call. This matches queries_debug.
• The env var is parsed for truthiness (%w[1 true yes on].include?) rather than taken raw. The neighbouring queries_debug line uses the raw form, where QUERIES_DEBUG=false is a non-empty String and therefore turns debug on; this is the same trap already called out in the query_logging comment. That pre-existing line is left alone here.
• The error-path puts calls in the same method are deliberately untouched. They report real failures and must not be gated behind a debug flag. Whether they belong on stderr (warn, as resilience.rb and cache.rb already do) is a separate question, intentionally left out of this PR.

Verification

• flag defaults to false, and toggles correctly
• env parsing accepts 1 / true / on / YES; rejects false / FALSE / 0 / no / unset
• calling append_triples_batch with a stubbed network call prints nothing when off, and exactly one line when on
• config.rb.sample loads and yields false as shipped, true with the line uncommented
• test/test_cache_unit.rb passes (17 tests, 43 assertions, 0 failures)

No automated test accompanies the flag; happy to add one covering the env parsing if reviewers would prefer.

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