streaming-standardization: add stream-based process + process_file convenience

[rferreira]

Summary

Implements the streaming standardization for scanii-ruby per STREAMING_STANDARDIZATION.md §3.8.

What changed

• New canonical methods: process(io, filename:, ...) and process_async(io, filename:, ...) accept any IO-like object (duck-typed on read(n)). Both File and StringIO work.
• New path convenience methods: process_file(path, ...) and process_async_file(path, ...) open the file in binary mode and delegate to the stream-based methods. These replace the old process(path) form.
• Deprecated aliases: process(String_path, ...) and process_async(String_path, ...) still work but emit a runtime warn directing callers to process_file / process_async_file. Will be removed in a future major version.
• True streaming: lib/scanii/multipart.rb refactored to stream_encode — builds prologue + epilogue as binary Strings, chains them around the caller's IO via a ChainedIO class, and sends the body via Net::HTTP#body_stream= with an explicit Content-Length. File content is never fully buffered.

Version bump: 1.0.1 → 1.1.0

Per API_VERSIONING_POLICY.md: the change shape is additive (new public methods, no removals, deprecated aliases kept). Additive changes ship as minor version bumps. No existing consumer code breaks; deprecated callers get a warn but keep working.

Tests

• Unit: StringIO path, File.open IO path, process_file path, deprecated process(path) emits warning, process_async variants
• Integration: all above plus live scanii-cli runs confirming end-to-end streaming works
• 57 runs, 135 assertions, 0 failures, 3 expected skips (UUID fixture + callback require newer scanii-cli build — self-skip per workspace convention)
• rubocop: no offenses

Test plan

• CI passes on Linux / macOS / Windows × Ruby 3.4 + 4.0
• Scan a real file via process_file — result is clean or flagged as expected
• Scan a StringIO via process(StringIO.new(...), filename: "test.bin") — same result
• Call old process(path) — confirm deprecation warning appears on stderr

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