Add test coverage report and 95% target

[clintjeff2]

chore: add test coverage report and 95% target

Summary

Implements issue #53 — adds cargo-tarpaulin based test coverage measurement
and enforces a minimum of 95 % line coverage on every push and pull-request.

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Changes

New files

File	Purpose
tarpaulin.toml	cargo-tarpaulin configuration — fail-under = 95, HTML + XML + Stdout reports, lib = true to avoid Soroban cdylib linker issues
scripts/coverage.sh	One-command local coverage runner; auto-installs cargo-tarpaulin if missing
.github/workflows/coverage.yml	CI workflow: runs on push / PR, uploads HTML artefact, posts coverage table as a PR comment

Modified files

File	Change
contracts/vault/src/test.rs	Expanded from 2 to 8 tests — added cases for None initial balance, uninitialised vault, insufficient-balance guard, exact-balance deduction, and get_meta field validation
README.md	New Test coverage section documents local and CI usage
.gitignore	Added coverage/ to keep generated HTML / XML out of git

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Coverage result

100 % line coverage (24 / 24 lines) — exceeds the 95 % minimum.

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How to reproduce locally

# One-time setup + run
./scripts/coverage.sh

# Or directly
cargo tarpaulin --config tarpaulin.toml

Open coverage/tarpaulin-report.html in a browser for the interactive report.

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CI behaviour

• cargo tarpaulin --config tarpaulin.toml runs on every push to main / master and on every PR.
• The workflow exits non-zero (failing the required check) if coverage drops below 95 %.
• The HTML report is uploaded as a downloadable workflow artefact (kept 30 days).
• A summary table is posted as a comment on every pull-request.

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Checklist

• All 8 tests pass (cargo test)
• Coverage ≥ 95 % verified locally (cargo tarpaulin)
• tarpaulin.toml parses without warnings
• scripts/coverage.sh is executable and self-contained
• GitHub Actions workflow added
• README updated with coverage section
• coverage/ added to .gitignore
• Screenshot of passing coverage run attached above

Closes #53

[greatest0fallt1me]

@clintjeff2 please resolve the conflicts

[clintjeff2]

@greatest0fallt1me
Please merge this ASAP, other contributors messed up the test files, had to redo everything all over again. And it was even more difficult.

[greatest0fallt1me]

@clintjeff2 resolve the conflicts