fix(cgo): remove -L${SRCDIR} from LDFLAGS to eliminate ld warning for external consumers

[WGB5445]

Summary

• Remove -L${SRCDIR}/native/<triple> from all 8 platform CGO files — this path only resolved correctly for in-repo builds. External consumers saw a spurious ld: warning: search path '...go@v1.1.2/aptosconfidential/native/<triple>' not found because ${SRCDIR} expands to the read-only module cache where no .a file exists.
• Both in-repo developers and external consumers now pass the library search path explicitly via CGO_LDFLAGS.
• CI, README, and CLAUDE.md updated to reflect the new requirement.

Usage after this change

External consumer:

go run github.com/aptos-labs/confidential-asset-bindings/bindings/go/aptosconfidential/tools/download@v1.1.3
CGO_LDFLAGS="-L$(pwd)/native/aarch64-apple-darwin" go build ./...

In-repo developer:

cargo build -p aptos_confidential_asset_ffi --release --manifest-path rust/Cargo.toml
TRIPLE=aarch64-apple-darwin
mkdir -p bindings/go/aptosconfidential/native/$TRIPLE
cp rust/target/release/libaptos_confidential_asset_ffi.a bindings/go/aptosconfidential/native/$TRIPLE/
cd bindings/go
CGO_LDFLAGS="-L$(pwd)/aptosconfidential/native/$TRIPLE" go test ./aptosconfidential/...

Test plan

• In-repo: CGO_LDFLAGS="-L$(pwd)/aptosconfidential/native/aarch64-apple-darwin" go test ./aptosconfidential/... — PASS, zero ld warnings
• External consumer (via local replace): CGO_LDFLAGS="-L$(pwd)/native/aarch64-apple-darwin" go build ./... — PASS, zero ld warnings
• CI go-bindings-smoke (linux/amd64) passes with updated CGO_LDFLAGS env var

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