Fix initial runahead limit, for offset recurrence start

[hjoliver]

Close #5705

When computing the runahead limit at start-up (before the task pool is loaded), recurrence start points beyond the limit have to be ignored.

Getting this right for "max cycles" limits (e.g. P3 in integer or datetime cycling, as opposed to PT3H in datetime cycling) requires counting sequence points for all sequences.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• CHANGES.md entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[wxtim]

• Reproduced Bug
• Read changes
• Checked out new tests - ensure failure against master.
• Checked out whole branch - test pass, played with code.