docs: add contract authorization matrix

[maxw06]

Closes #1479.

Summary

• Adds docs/AUTH_MATRIX.md, a source-grounded authorization matrix for exported QuickLendXContract entrypoints.
• Links the matrix from the top-level README and the existing access-control documentation.
• Calls out signer-enforced, identity-only, state-gated, public-read, and exported internal-style helper patterns so reviewers can audit the current role surface.

Validation

git diff --check HEAD~1 HEAD
ruby <<'RUBY'
path='quicklendx-contracts/src/lib.rs'
text=File.read(path)
text = text.gsub(%r{/\*.*?\*/}m, '')
text = text.lines.map { |l| l.sub(%r{//.*$}, '') }.join
funcs = text.scan(/^\s*pub fn\s+(\w+)/).flatten.uniq
doc = File.read('docs/AUTH_MATRIX.md')
missing = funcs.reject { |f| doc.include?("`#{f}`") }
puts "exported_pub_fn_count=#{funcs.length}"
puts "missing_count=#{missing.length}"
puts missing.join("\n") unless missing.empty?
RUBY
cargo test -p quicklendx-contracts --doc

git diff --check passed and the matrix coverage check reported missing_count=0.

cargo test -p quicklendx-contracts --doc currently fails on upstream main compile debt unrelated to this docs-only change: duplicate bench and compute_yield definitions, missing idempotency / address_summary modules, and an unhandled InvalidLedgerSequence error arm.

[Baskarayelu]

thanks for the contribution! our workflow only merges a PR when its author is assigned to the linked issue — please get assigned to an open issue first, then raise the PR. closing as invalid for now 🙏