docs: update the SECURITY.md supported-versions table to the 0.7.x line #1630

Summary

SECURITY.md still describes 0.5.x as the current stable line, two release lines behind reality. Cargo.toml is at 0.7.0-beta.1, and both v0.6.0 (2026-08-22) and v0.7.0-beta.1 (2026-09-04) have shipped per the CHANGELOG and git tags.

Background

• SECURITY.md:11-16 lists 0.5.x as "Current stable line", and the note below reads "mlxcel's current release line is 0.5.x (tags v0.1.4 through v0.5.2 have shipped)". Both confirmed in the current file.
• Cargo.toml:32 sets version = "0.7.0-beta.1".
• CHANGELOG.md:7 (v0.7.0-beta.1, 2026-09-04) and CHANGELOG.md:76 (v0.6.0, 2026-08-22).

Proposed Solution

Update the supported-versions table and the prose note to reflect the 0.7.x line, and the support policy the maintainers want for the 0.6.x and 0.5.x lines.

Implementation Notes

• The current values are mechanically derivable from Cargo.toml, git tag, and the CHANGELOG.
• The support-policy decision for older lines is a maintainer call; the mechanical facts (current line, latest shipped tag) are not.

Acceptance Criteria

• The supported-versions table names the actual current release line.
• The prose note lists the actual latest shipped tag.

Original Suggestion

Title: docs: update the SECURITY.md supported-versions table to the 0.7.x line

SECURITY.md still describes 0.5.x as the current stable line, two release lines behind reality. Cargo.toml is at 0.7.0-beta.1, and both v0.6.0 (2026-08-22) and v0.7.0-beta.1 (2026-09-04) have shipped per the CHANGELOG and git tags.

Evidence

• SECURITY.md:11-16 — the supported-versions table lists 0.5.x as "Current stable line", and the note below reads "mlxcel's current release line is 0.5.x (tags v0.1.4 through v0.5.2 have shipped)"
• Cargo.toml:33 — version = "0.7.0-beta.1"
• CHANGELOG.md:7 (v0.7.0-beta.1, 2026-09-04) and CHANGELOG.md:76 (v0.6.0, 2026-08-22)

Suggested fix

Update the table and the note to reflect the 0.7.x line (and whatever support policy the maintainers want for 0.6.x/0.5.x). The correct values are mechanically derivable from Cargo.toml, git tag, and the CHANGELOG.

Acceptance criteria

• The supported-versions table names the actual current release line
• The prose note lists the actual latest shipped tag