feat(sprintf): support full %q format specifications

[Vitalii Tverdokhlib (vitaliytv)]

Depends on #809.

This follow-up completes %q support with a dedicated sprintf format-spec parser instead of extending the original PR's single-verb parsing incrementally.

What this adds

• Parses Go-style flags, width, precision, dynamic * operands, and explicit argument indexes such as %[3]*.[2]*[1]q.
• Supports %+q (QuoteToASCII behavior), %#q (backquoted strings when valid), left/right alignment, zero padding, and combined width/precision.
• Applies width and precision in Unicode code points, matching Go/OPA rather than UTF-8 byte length.
• Handles %q operands using OPA-compatible value conversion for strings, booleans, null, collections, integral runes, invalid runes, fractional numbers, and out-of-range integers.
• Preserves the existing behavior of non-q verbs. Newly parsed flags that those verbs do not support are rejected explicitly.
• Keeps width and precision bounded by the existing memory limit strategy.

Compatibility boundary

Regorus currently normalizes numbers into its internal Number variants and does not retain the original Rego numeric token spelling. OPA's %q behavior can depend on that spelling: for example, OPA distinguishes 65.0, 65e0, and 65, while Regorus may normalize them to different shared numeric variants. Exact parity for those lexical-number edge cases requires a separate change to the number representation; this PR does not disguise or broaden that architectural change.

The common %q matrix and parser edge cases were compared directly with OPA, including ASCII quoting, raw quoting and fallback, width/precision combinations, dynamic and indexed operands, runes, invalid runes, collections, and diagnostics.

Validation

• Full repository pre-push suite passed.
• cargo test: 298 passed, 2 ignored.
• OPA compatibility suite: 2875 passed, 0 failed.
• cargo test --locked --no-default-features --lib: 220 passed, 2 ignored.
• cargo xtask test-no-std passed.
• cargo clippy --locked --all-targets --all-features -- -D warnings passed.

This is intentionally stacked on #809. Until #809 is merged, GitHub's diff against main includes that prerequisite commit as well; afterward this PR reduces to the format-spec extension.

[Anand Krishnamoorthi (anakrish)]

Vitalii Tverdokhlib (@vitaliytv) Thank you for the contribution. Going to take a deeper look at this today/tomorrow.

[Copilot]

🟡 Changes recommended

Format compatibility gaps and unchecked allocation paths can produce incorrect output or substantially exceed configured memory limits.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Completes Go/OPA-compatible %q formatting for sprintf.

Changes:

• Adds format-spec parsing, quoting flags, dynamic/indexed operands, and value conversions.
• Ports Go Unicode printability tables.
• Adds compatibility and memory-limit tests.

File summaries

File	Description
src/builtins/strings.rs	Implements %q formatting and integration.
src/builtins/strings/sprintf_format.rs	Parses Go-style format specifications.
src/builtins/strings/go_is_print.rs	Provides Go-compatible Unicode classification.
tests/interpreter/cases/builtins/strings/sprintf.yaml	Adds interpreter compatibility cases.
tests/memory_limits.rs	Tests quoting memory-limit propagation.
LICENSE	Attributes Go-derived source.

Review details

Suppressed comments (1)

src/builtins/strings/sprintf_format.rs:119

• A negative dynamic precision must produce Go's %!(BADPREC) diagnostic and then format without precision. With sprintf("%.*q", [-1, "x"]), this silently returns "x" instead of %!(BADPREC)"x", so a valid integer * operand has incompatible OPA behavior. Preserve a bad-precision marker in the parsed spec and emit it before formatting the value.

            let precision = take_integer(args, args_idx, args_span)?;
            if precision >= 0 {
                spec.precision = Some(checked_dynamic_value(precision as u64, args_span)?);
            }

• Files reviewed: 6/6 changed files
• Comments generated: 7
• Review effort level: Balanced

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[Vitalii Tverdokhlib (vitaliytv)]

Also addressed the suppressed review note in 3568dac: negative dynamic precision now emits Go/OPA-compatible %!(BADPREC) and then formats the value without precision. Added an interpreter regression case for %.*q with -1.